Unify X and layers

[flying-sheep]

• Closes Unification of X and layers #244
• Tests added
• Release note added (or unnecessary)

TODO:

• decide on dtype arg: just remove? or ignore+warning? (unless we already do that, then remove)
• backed mode: replace with Dask?

[codecov]

Codecov Report

Attention: Patch coverage is 95.94595% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.01%. Comparing base (437dbc8) to head (a6555f8).

Files with missing lines	Patch %	Lines
src/anndata/_core/anndata.py	88.88%	2 Missing ⚠️
src/anndata/_core/aligned_mapping.py	97.82%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (437dbc8) and HEAD (a6555f8). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (437dbc8)	HEAD (a6555f8)
	3	2

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1707      +/-   ##
==========================================
- Coverage   86.93%   80.01%   -6.92%     
==========================================
  Files          40       40              
  Lines        6039     5985      -54     
==========================================
- Hits         5250     4789     -461     
- Misses        789     1196     +407     

Files with missing lines	Coverage Δ
src/anndata/_core/raw.py	80.85% <100.00%> (-0.45%)	⬇️
src/anndata/_io/h5ad.py	93.00% <100.00%> (ø)
src/anndata/_io/specs/methods.py	88.03% <100.00%> (-0.43%)	⬇️
src/anndata/tests/helpers.py	82.98% <100.00%> (-11.38%)	⬇️
src/anndata/_core/aligned_mapping.py	93.67% <97.82%> (+0.26%)	⬆️
src/anndata/_core/anndata.py	82.46% <88.88%> (-1.27%)	⬇️

... and 11 files with indirect coverage changes