lazy dataframes in .obs and .var with backed="r" mode

[gszep]

closes #981

[codecov]

Codecov Report

Merging #983 (846d07e) into main (dcee1e5) will decrease coverage by 0.32%.
The diff coverage is 46.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #983      +/-   ##
==========================================
- Coverage   84.15%   83.84%   -0.32%     
==========================================
  Files          34       35       +1     
  Lines        4709     4734      +25     
==========================================
+ Hits         3963     3969       +6     
- Misses        746      765      +19     

Impacted Files	Coverage Δ
anndata/_core/dataframe.py	39.13% <39.13%> (ø)
anndata/_core/anndata.py	82.31% <100.00%> (-0.36%)	⬇️
anndata/_io/specs/methods.py	87.81% <100.00%> (+0.03%)	⬆️

... and 2 files with indirect coverage changes