Fix bug with probeset and index mismatch when using cellranger multi with a probeset and without a pre-built index

Address https://github.com/nf-core/scrnaseq/issues/375. I chose to enforce that the user manually pass the genome version to keep some sort of front-loaded check on the reference matching the probeset. Updated tests accordingly. ## PR checklist - [x] This comment contains a description of changes (with reason). - [x] If you've fixed a bug or added code that should be tested, add tests! - [x] Make sure your code lints (`nf-core pipelines lint`). - [x] Ensure the test suite passes (`nextflow run . -profile test,docker --outdir <OUTDIR>`). - [x] Check for unexpected warnings in debug mode (`nextflow run . -profile debug,test,docker --outdir <OUTDIR>`). - [x] Usage Documentation in `docs/usage.md` is updated. - [x] `CHANGELOG.md` is updated.