scGPT cell classification workflow

[jakubmajercik]

Changelog

Implementation of workflow for cell type classification with scGPT

Issue ticket number and link

Closes #777

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

• [x] CI tests succeed!