subsetting + views of MuData/AnnData

[ilia-kats]

We want to be able to write mdata[1:10, 5] and get a corresponding MuData object back, as in the Python implementation. One point of discussion is the handling of views. In contrast to NumPy, Julia does not create views by default, one has to request one explicitly with @view or @views. I think it makes sense to adopt this behavior for Muon to be consistent with the rest of the Julia ecosystem.

[ilia-kats]

Most stuff should be working now, closing. Bugs in the existing implementation or missing features should get new issues.