Full accessor support

[ilia-kats]

Implement MuData acessors. These are similar to and build on AnnData accessors, but add an additional level for modalities. So it is now possible to e.g. do

from mudata.acc import A

acc = A.mod["rna"].obs["cell_type"]

mdata[acc]

This builds on #147, so that one should go in first.

[codecov]

Codecov Report

❌ Patch coverage is 87.67123% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.92%. Comparing base (1837822) to head (5ccd578).

Files with missing lines	Patch %	Lines
src/mudata/acc/__init__.py	86.17%	26 Missing ⚠️
src/mudata/_core/mudata.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
- Coverage   91.38%   90.92%   -0.46%     
==========================================
  Files          10       11       +1     
  Lines        1601     1819     +218     
==========================================
+ Hits         1463     1654     +191     
- Misses        138      165      +27     

Files with missing lines	Coverage Δ
src/mudata/__init__.py	100.00% <100.00%> (ø)
src/mudata/_core/mudata.py	93.01% <96.29%> (+0.09%)	⬆️
src/mudata/acc/__init__.py	86.17% <86.17%> (ø)

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