new 10x mtx version

Hi,
bc.export.X_to_mtx() output the previous mtx generated by cellranger 2:
barcodes.tsv genes.tsv matrix.mtx

The latest cellranger generated distinct outputs now:
barcodes.tsv.gz features.tsv.gz matrix.mtx.gz

"genes.tsv" in cellranger2 has become "features.tsv.gz" in cellranger3. There are two columns in genes.tsv but three columns in features.tsv.gz. Could you update bc.export.X_to_mtx() to support the latest mtx ?

Start by locating bc.export.X_to_mtx() and inspect how it handles the Cell Ranger 2 files barcodes.tsv, genes.tsv, and matrix.mtx. Verify support for the Cell Ranger 3 files barcodes.tsv.gz, features.tsv.gz with three columns, and matrix.mtx.gz; done means the latest mtx output is accepted.

Written by the indexing model from the issue text.