ExperimentAxisQuery - to_anndata obsp/varp as sparse matrices

[bkmartinjr]

Issue and/or context:

Fixes #3360

Changes:

obsp/varp slots in the AnnData returned by ExperimentAxisQuery.to_anndata will be sparse matrices. Previously they were dense NDArray.

Notes for Reviewer:

1. this required a bit of refactoring
2. as part of this, obsp/varp will be read using the partitioned reader (same as X layers)

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.97%. Comparing base (d1178ae) to head (83e778f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3387      +/-   ##
==========================================
+ Coverage   85.87%   85.97%   +0.09%     
==========================================
  Files          55       55              
  Lines        6143     6144       +1     
==========================================
+ Hits         5275     5282       +7     
+ Misses        868      862       -6     

Flag	Coverage Δ
python	85.97% <97.14%> (+0.09%)	⬆️

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Components	Coverage Δ
python_api	85.97% <97.14%> (+0.09%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

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