allow BAM files as input to the WASP subworkflow

The WASP subworkflow requires RNA-seq FASTQ files as input. But what if the alignment step has already been performed, and the user only has BAM files, instead?

We could automatically skip the first mapping step and go right to `find_intersecting_SNPs`. To ensure the second mapping step is performed identically to the first, we can add a config option allowing the user to provide the argument for the second mapping step or we could try to take it from the `@PG` tags in the BAM header.

Start at the WASP subworkflow and its find_intersecting_SNPs entry point; inspect how FASTQ inputs and the first mapping step are currently wired. Decide whether the second mapping argument should come from a config option or BAM @PG tags, then verify that BAM input skips the initial mapping while preserving identical second-step arguments.

Written by the indexing model from the issue text.