Added approximate KNN backend

[nicolassidoux]

I had to from scanpy.neighbors._types import KnnTransformerLike, _KnownTransformer to make transformer argument of neighbors() consistent with scanpy. But _types is protected...

[eroell]

@Zethson what is your stance on importing private variables from scanpy, as we are doing elsewhere too?

I remember in the past you've added a fix of a previously imported, external private variable by defining the required variable ourselves.

I lean towards doing this here too

[eroell]

The link breaks in our documentation as this was from the scanpy doc; but we could insert the full scanpy doc link instead https://scanpy.readthedocs.io/en/latest/how-to/knn-transformers.html & make a quick comment that this is scanpy but works the same for ehrapy

[eroell]

Here can either add "deprecated since scanpy version 1.10.0" such that its clear where 1.10.0 comes from, or hide this from the beginning as its deprecated already?
Then, would also need to remove from _KnownTransformer if we write this ourselves see question on what to do about private variables below

[Zethson]

@Zethson what is your stance on importing private variables from scanpy, as we are doing elsewhere too?

I remember in the past you've added a fix of a previously imported, external private variable by defining the required variable ourselves.

I lean towards doing this here too

Yeah, we should try to avoid importing internals so I think that's a good option.

[eroell]

Thank you so much @nicolassidoux !
One thing I realized here was that we should add tests for the pass through function calls of scanpy...

But this is from our side so far a lacking suite of tests (as we as of now have been closely eyeing scanpy), and hence no test suite at this point to add to here :)

[eroell]

ready for me to take a quick look again or are you still doing stuff? :)