(fix): disallow using dataframes with multi index columns

[ilan-gold]

• Closes Compatibility with pd.MultiIndex dimension labels #1202
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (07ca87c) to head (f43332f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/anndata/_core/aligned_mapping.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1589      +/-   ##
==========================================
- Coverage   86.70%   84.25%   -2.46%     
==========================================
  Files          37       37              
  Lines        5974     5989      +15     
==========================================
- Hits         5180     5046     -134     
- Misses        794      943     +149     

Files	Coverage Δ
src/anndata/_core/anndata.py	83.72% <100.00%> (+0.09%)	⬆️
src/anndata/_core/storage.py	100.00% <100.00%> (ø)
src/anndata/tests/helpers.py	81.85% <100.00%> (-12.21%)	⬇️
src/anndata/utils.py	83.74% <100.00%> (-0.68%)	⬇️
src/anndata/_core/aligned_mapping.py	93.41% <81.81%> (+0.05%)	⬆️

... and 6 files with indirect coverage changes

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 0.10.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 229e2a6228547d2ce5257373af3461614b666796

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #1589: (fix): disallow using dataframes with multi index columns'

4. Push to a named branch:

git push YOURFORK 0.10.x:auto-backport-of-pr-1589-on-0.10.x

5. Create a PR against branch 0.10.x, I would have named this PR:

"Backport PR #1589 on branch 0.10.x ((fix): disallow using dataframes with multi index columns)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.