`filter_with_hvg`: enable storing the number of highly variable genes found

[DriesSchaumont]

Changelog

filter_with_hvg: enable storing the number of highly variable genes found

Issue ticket number and link

Closes #595 (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
  • [x] New functionality
  • [ ] Major changes
  • [ ] Minor changes
  • [ ] Documentation
  • [ ] Bug fixes

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

• [ ] CI tests succeed!

[DriesSchaumont]

Closing because it is unclear how to handle this (and its easy to calculate the metric on the spot from the boolean column anyway)