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)