Remove `sc.external`

[flying-sheep]

External has outlived its usefulness with the emergence of the scverse ecosystem, and should be removed.

^ This text should be expanded

TODO

• [ ] Finalize methods to keep/ merge into main namespace
  • Currently: scanorama, hashsolo, scrublet or equivalent, maybe bbknn
• [ ] Make removal plan (probably deprecation message + maybe email to authors) for
  • [ ] harmony
  • [ ] dca
  • [ ] magic
  • [ ] phate
  • [ ] palantir
  • [ ] trimap
  • [ ] sam
  • [ ] phenograph
  • [ ] wishbone
  • [ ] sandbag
  • [ ] cyclone
  • [ ] Export functionality for
    • [ ] spring_project
    • [ ] cellbrowser

[flying-sheep]

@Intron7 you said you have a sense which ones are still used.

Could you please weigh in here?

[flying-sheep]

TODO: create sub-issues for all tools and invite feedback by maintainers and users

• what’s still maintained (and therefore people should use the upstream one)
• what’s unmaintained but still used (and therefore we should take over)