Fix validation and scale accessor multiscale image

[LucaMarconato]

Small PR adding the spatialdata.get_pyramid_levels() util function.

closes #106 (was mostly implemented already)
closes #355

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.86%. Comparing base (b2c34c2) to head (adead33).
Report is 41 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/models/models.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
+ Coverage   91.75%   91.86%   +0.11%     
==========================================
  Files          45       45              
  Lines        6898     6883      -15     
==========================================
- Hits         6329     6323       -6     
+ Misses        569      560       -9     

Files with missing lines	Coverage Δ
src/spatialdata/__init__.py	96.29% <100.00%> (ø)
src/spatialdata/_io/_utils.py	86.90% <100.00%> (-0.18%)	⬇️
src/spatialdata/_io/io_raster.py	96.03% <100.00%> (+0.03%)	⬆️
src/spatialdata/_utils.py	87.40% <100.00%> (+5.13%)	⬆️
src/spatialdata/models/models.py	87.82% <50.00%> (ø)