Generalize Barcode Platform r1 and i7 Inputs

[benjamincarlin]

The inputs into the attach barcode method in the Barcode Platform class include a read 1 fastq (r1), where the cell and molecule barcodes are extracted and an index fastq file (i7, optional), where the sample barcode is distracted.

sctools/src/sctools/platform.py

Line 865 in 09eb7f4

parser.add_argument('--r1',

In order to generalize this class further, is it possible to omit the numerical assignments to the reads. r1 and i1 could become a list of one or more input fastq files, and the tool could enable a user to specify which fastq file the tag should be associated with. These suggestions are from @ambrosejcarr comments and past experiences with In-drop and Mars-seq, where the barcodes are not always in the same positions or on the same fastq files. Please see the following PR where this was discussed...

#57