Support matrix.mtx.gz cellbender remove-background input

[alecw]

cellbender remove-background can use as input the file triplet matrix.mtx.gz, barcodes.tsv.gz, features.tsv.gz. This change enables this input format via:

• passing the 3 files in the h5ad input parameter;
• and setting task.ext.input = '.', which tells remove-background to look for the 3 files in the working directory.

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