--- a
+++ b/docs/pkgdown.css
@@ -0,0 +1,384 @@
+/* Sticky footer */
+
+/**
+ * Basic idea: https://philipwalton.github.io/solved-by-flexbox/demos/sticky-footer/
+ * Details: https://github.com/philipwalton/solved-by-flexbox/blob/master/assets/css/components/site.css
+ *
+ * .Site -> body > .container
+ * .Site-content -> body > .container .row
+ * .footer -> footer
+ *
+ * Key idea seems to be to ensure that .container and __all its parents__
+ * have height set to 100%
+ *
+ */
+
+html, body {
+  height: 100%;
+}
+
+body {
+  position: relative;
+}
+
+body > .container {
+  display: flex;
+  height: 100%;
+  flex-direction: column;
+}
+
+body > .container .row {
+  flex: 1 0 auto;
+}
+
+footer {
+  margin-top: 45px;
+  padding: 35px 0 36px;
+  border-top: 1px solid #e5e5e5;
+  color: #666;
+  display: flex;
+  flex-shrink: 0;
+}
+footer p {
+  margin-bottom: 0;
+}
+footer div {
+  flex: 1;
+}
+footer .pkgdown {
+  text-align: right;
+}
+footer p {
+  margin-bottom: 0;
+}
+
+img.icon {
+  float: right;
+}
+
+/* Ensure in-page images don't run outside their container */
+.contents img {
+  max-width: 100%;
+  height: auto;
+}
+
+/* Fix bug in bootstrap (only seen in firefox) */
+summary {
+  display: list-item;
+}
+
+/* Typographic tweaking ---------------------------------*/
+
+.contents .page-header {
+  margin-top: calc(-60px + 1em);
+}
+
+dd {
+  margin-left: 3em;
+}
+
+/* Section anchors ---------------------------------*/
+
+a.anchor {
+  display: none;
+  margin-left: 5px;
+  width: 20px;
+  height: 20px;
+
+  background-image: url(./link.svg);
+  background-repeat: no-repeat;
+  background-size: 20px 20px;
+  background-position: center center;
+}
+
+h1:hover .anchor,
+h2:hover .anchor,
+h3:hover .anchor,
+h4:hover .anchor,
+h5:hover .anchor,
+h6:hover .anchor {
+  display: inline-block;
+}
+
+/* Fixes for fixed navbar --------------------------*/
+
+.contents h1, .contents h2, .contents h3, .contents h4 {
+  padding-top: 60px;
+  margin-top: -40px;
+}
+
+/* Navbar submenu --------------------------*/
+
+.dropdown-submenu {
+  position: relative;
+}
+
+.dropdown-submenu>.dropdown-menu {
+  top: 0;
+  left: 100%;
+  margin-top: -6px;
+  margin-left: -1px;
+  border-radius: 0 6px 6px 6px;
+}
+
+.dropdown-submenu:hover>.dropdown-menu {
+  display: block;
+}
+
+.dropdown-submenu>a:after {
+  display: block;
+  content: " ";
+  float: right;
+  width: 0;
+  height: 0;
+  border-color: transparent;
+  border-style: solid;
+  border-width: 5px 0 5px 5px;
+  border-left-color: #cccccc;
+  margin-top: 5px;
+  margin-right: -10px;
+}
+
+.dropdown-submenu:hover>a:after {
+  border-left-color: #ffffff;
+}
+
+.dropdown-submenu.pull-left {
+  float: none;
+}
+
+.dropdown-submenu.pull-left>.dropdown-menu {
+  left: -100%;
+  margin-left: 10px;
+  border-radius: 6px 0 6px 6px;
+}
+
+/* Sidebar --------------------------*/
+
+#pkgdown-sidebar {
+  margin-top: 30px;
+  position: -webkit-sticky;
+  position: sticky;
+  top: 70px;
+}
+
+#pkgdown-sidebar h2 {
+  font-size: 1.5em;
+  margin-top: 1em;
+}
+
+#pkgdown-sidebar h2:first-child {
+  margin-top: 0;
+}
+
+#pkgdown-sidebar .list-unstyled li {
+  margin-bottom: 0.5em;
+}
+
+/* bootstrap-toc tweaks ------------------------------------------------------*/
+
+/* All levels of nav */
+
+nav[data-toggle='toc'] .nav > li > a {
+  padding: 4px 20px 4px 6px;
+  font-size: 1.5rem;
+  font-weight: 400;
+  color: inherit;
+}
+
+nav[data-toggle='toc'] .nav > li > a:hover,
+nav[data-toggle='toc'] .nav > li > a:focus {
+  padding-left: 5px;
+  color: inherit;
+  border-left: 1px solid #878787;
+}
+
+nav[data-toggle='toc'] .nav > .active > a,
+nav[data-toggle='toc'] .nav > .active:hover > a,
+nav[data-toggle='toc'] .nav > .active:focus > a {
+  padding-left: 5px;
+  font-size: 1.5rem;
+  font-weight: 400;
+  color: inherit;
+  border-left: 2px solid #878787;
+}
+
+/* Nav: second level (shown on .active) */
+
+nav[data-toggle='toc'] .nav .nav {
+  display: none; /* Hide by default, but at >768px, show it */
+  padding-bottom: 10px;
+}
+
+nav[data-toggle='toc'] .nav .nav > li > a {
+  padding-left: 16px;
+  font-size: 1.35rem;
+}
+
+nav[data-toggle='toc'] .nav .nav > li > a:hover,
+nav[data-toggle='toc'] .nav .nav > li > a:focus {
+  padding-left: 15px;
+}
+
+nav[data-toggle='toc'] .nav .nav > .active > a,
+nav[data-toggle='toc'] .nav .nav > .active:hover > a,
+nav[data-toggle='toc'] .nav .nav > .active:focus > a {
+  padding-left: 15px;
+  font-weight: 500;
+  font-size: 1.35rem;
+}
+
+/* orcid ------------------------------------------------------------------- */
+
+.orcid {
+  font-size: 16px;
+  color: #A6CE39;
+  /* margins are required by official ORCID trademark and display guidelines */
+  margin-left:4px;
+  margin-right:4px;
+  vertical-align: middle;
+}
+
+/* Reference index & topics ----------------------------------------------- */
+
+.ref-index th {font-weight: normal;}
+
+.ref-index td {vertical-align: top; min-width: 100px}
+.ref-index .icon {width: 40px;}
+.ref-index .alias {width: 40%;}
+.ref-index-icons .alias {width: calc(40% - 40px);}
+.ref-index .title {width: 60%;}
+
+.ref-arguments th {text-align: right; padding-right: 10px;}
+.ref-arguments th, .ref-arguments td {vertical-align: top; min-width: 100px}
+.ref-arguments .name {width: 20%;}
+.ref-arguments .desc {width: 80%;}
+
+/* Nice scrolling for wide elements --------------------------------------- */
+
+table {
+  display: block;
+  overflow: auto;
+}
+
+/* Syntax highlighting ---------------------------------------------------- */
+
+pre, code, pre code {
+  background-color: #f8f8f8;
+  color: #333;
+}
+pre, pre code {
+  white-space: pre-wrap;
+  word-break: break-all;
+  overflow-wrap: break-word;
+}
+
+pre {
+  border: 1px solid #eee;
+}
+
+pre .img, pre .r-plt {
+  margin: 5px 0;
+}
+
+pre .img img, pre .r-plt img {
+  background-color: #fff;
+}
+
+code a, pre a {
+  color: #375f84;
+}
+
+a.sourceLine:hover {
+  text-decoration: none;
+}
+
+.fl      {color: #1514b5;}
+.fu      {color: #000000;} /* function */
+.ch,.st  {color: #036a07;} /* string */
+.kw      {color: #264D66;} /* keyword */
+.co      {color: #888888;} /* comment */
+
+.error   {font-weight: bolder;}
+.warning {font-weight: bolder;}
+
+/* Clipboard --------------------------*/
+
+.hasCopyButton {
+  position: relative;
+}
+
+.btn-copy-ex {
+  position: absolute;
+  right: 0;
+  top: 0;
+  visibility: hidden;
+}
+
+.hasCopyButton:hover button.btn-copy-ex {
+  visibility: visible;
+}
+
+/* headroom.js ------------------------ */
+
+.headroom {
+  will-change: transform;
+  transition: transform 200ms linear;
+}
+.headroom--pinned {
+  transform: translateY(0%);
+}
+.headroom--unpinned {
+  transform: translateY(-100%);
+}
+
+/* mark.js ----------------------------*/
+
+mark {
+  background-color: rgba(255, 255, 51, 0.5);
+  border-bottom: 2px solid rgba(255, 153, 51, 0.3);
+  padding: 1px;
+}
+
+/* vertical spacing after htmlwidgets */
+.html-widget {
+  margin-bottom: 10px;
+}
+
+/* fontawesome ------------------------ */
+
+.fab {
+    font-family: "Font Awesome 5 Brands" !important;
+}
+
+/* don't display links in code chunks when printing */
+/* source: https://stackoverflow.com/a/10781533 */
+@media print {
+  code a:link:after, code a:visited:after {
+    content: "";
+  }
+}
+
+/* Section anchors ---------------------------------
+   Added in pandoc 2.11: https://github.com/jgm/pandoc-templates/commit/9904bf71
+*/
+
+div.csl-bib-body { }
+div.csl-entry {
+  clear: both;
+}
+.hanging-indent div.csl-entry {
+  margin-left:2em;
+  text-indent:-2em;
+}
+div.csl-left-margin {
+  min-width:2em;
+  float:left;
+}
+div.csl-right-inline {
+  margin-left:2em;
+  padding-left:1em;
+}
+div.csl-indent {
+  margin-left: 2em;
+}