cellranger multi: make the lane optional

[rcannood]

Changelog

• mapping/cellranger_multi: Fix the regex for the fastq input files to allow dropping the lane from the input file names (e.g. _L001).

Issue ticket number and link

Closes #xxxx (Replace xxxx with the GitHub issue number)

Checklist before requesting a review

• [x] I have performed a self-review of my code

• [x] Conforms to the Contributor's guide

• Check the correct box. Does this PR contain:

  • [ ] Breaking changes
  • [ ] New functionality
  • [ ] Major changes
  • [ ] Minor changes
  • [ ] Documentation
  • [x] Bug fixes

• [ ] Proposed changes are described in the CHANGELOG.md

• [ ] CI tests succeed!