basic support for AnnData accessors

[ilia-kats]

The main point of this is to still be able to pass MuData objects to scanpy's plotting functions. This is currently possible due to MuData's implementation of obs_vector and var_vector, but if I understand the scanpy changes correctly (e.g. here) this will stop working as soon as anndata 0.13 is released.

The logic is based on the current _attr_vector implementation, which will go away at some point when obs_vector and var_vector are removed.

I'd appreciate some feedback on where else scanpy has switched to accessors. For example, it is currently possible to run sc.pp.neighbors and sc.tl.umap on a MuData object. Have these also switched to using accessors to access .obsm/obsp?

[codecov]

Codecov Report

❌ Patch coverage is 7.69231% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.86%. Comparing base (61f35d2) to head (86fed09).

Files with missing lines	Patch %	Lines
src/mudata/_core/mudata.py	7.69%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
- Coverage   91.54%   90.86%   -0.69%     
==========================================
  Files          10       10              
  Lines        1585     1598      +13     
==========================================
+ Hits         1451     1452       +1     
- Misses        134      146      +12     

Files with missing lines	Coverage Δ
src/mudata/_core/mudata.py	92.09% <7.69%> (-1.30%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.