Switch to leiden

[flying-sheep]

see scverse/scanpy#1283 (comment)

@LuckyMD we should change the tutorials instead of going back, because louvain is deprecated:

This package has been superseded by the leidenalg package and will no longer be maintained. Please upgrade to the leidenalg package.

[LuckyMD]

I don't mind changing the defaults in the tutorials, but I think there's also a louvain function in the leidenalg package if you don't want to change defaults just yet, but just package reliance.

[flying-sheep]

Hmm, if we’re going to change something anyway, shouldn’t we just go with the better option?

[LuckyMD]

I think that's fine... we could however still wrap sc.tl.louvain to use the leidenalg implementation so that people's code still works.