Write cell ids into barcodes.tsv in the mtx output

[mckinsel]

barcodes.tsv is there for compatibility with Read10X methods. But we output data with no barcodes, and there may be duplicate barcodes in one output file. So instead of writing the barcode, write the cellkey, which always exists and is unique for every cell.

[mtmorgan]

If you're moving from emulating 10x, then perhaps just produce cells.tsv? for instance cells.tsv in https://data.humancellatlas.org/project-assets/project-matrices/cddab57b-6868-4be4-806f-395ed9dd635a.mtx.zip has both a 'cellkey' and 'barcode' column, where the barcode column is already empty (and the barcode.tsv file has blank lines!)

[mckinsel]

Hey @mtmorgan , we actually moving to emulating 10x outputs. We have users who want to get an mtx directory from the DCP and read it with, say, the Read10X function in Seurat. We used to only have cells.tsv but need to add barcodes if the existing methods are going to work.