pycombat_seq: batch effect correction for RNASeq data

ComBat-Seq [Zhang2020]_ follows on the steps of ComBat, but targets specifically RNA-Seq data. Conceptually, ComBat-Seq is based on the same mathematical framework as ComBat, except that its replaces the normal distribution of microarray data by a negative binomial distribution to account for the specificities of RNA-Seq expression data. :func:`pycombat_seq` is a direct port of ComBat-Seq to Python. Since ComBat-Seq relies on the Bioconductor edgeR package, the relevant parts of edgeR have been ported along. Closely following the original implementation in R, :func:`pycombat_seq` has results very similar to those of ComBat-Seq in terms of batch effects correction. Additionally, :func:`pycombat_seq` is as fast, if not faster, than the original implementation in R. It also features additional capabilities, such as fixing a given batch as reference.