Use GBCMS directly instead of genotype variants

[buehlere]

Instead of using genotype variants, It would be nice to use GBCMS directly. We can use local python module and nextflow channel manipulation to recreate the functionality of genotype variants. I think this would add more transparency to the calls being made by GBCMS.

[buehlere]

consider using traceback: https://github.com/mskcc-omics-workflows/modules/blob/develop/subworkflows/msk/traceback/main.nf