cellranger-multi implementation follow-up

[fmalmeida]

Description of feature

During the inclusion of cellranger-multi on #276 a few TODOs were left to be closed on follow-up PRs. These are:

• [ ] Include automated checkers for cellranger multi parameter combinations.
  • For example, when VDJ data is given, require VDJ ref. If FFPE, require frna probe sets, etc.
• [ ] validate multi-samplesheet using the same logic as the main samplesheet (using nf-validation)
• [ ] Assess possibility of adding automated "if-else" for the VDJ and GEX index reference building.
• [ ] Assess the inclusion and support of BEAM data
  • Assess execution of emptydrops to CRISPR/BEAM/ANTIBODY modes. Maybe is not relevant and can be filtered?
• [ ] Update cellrangermulti test profiles so that required ensembl reference files are available in nf-core test datasets.

FYI: @grst