Handle MEX datasets with barcode_metadata.tsv.gz files

[arteymix]

This file format is used for datasets that are aligned against a combined reference (i.e. hg19 + mm10). It contains additional columns with per-species counts and a call that indicate which species the cell is coming from.

Example: https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE218621

[arteymix]

I've added support for detecting those cases as MEX, but not download or load them. We will likely have to transform the file to extract barcodes relevant to the species indicated in the sample.