fix models docstrings

[LucaMarconato]

• fixed docstrings data model
• added an implementation of force_2d() utils function valid for GeoDataFrame constructed with ShapesModel.parse(). force_2d() is available in geopandas but not release yet and it's needed for Shift+E creates Polygon Z in some cases napari-spatialdata#218.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.53%. Comparing base (3b29f0b) to head (217bd4b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #530   +/-   ##
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  Coverage   92.53%   92.53%           
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  Files          43       43           
  Lines        5972     5991   +19     
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+ Hits         5526     5544   +18     
- Misses        446      447    +1     

Files	Coverage Δ
src/spatialdata/models/__init__.py	100.00% <ø> (ø)
src/spatialdata/models/models.py	87.99% <ø> (ø)
src/spatialdata/models/_utils.py	91.60% <94.73%> (+0.53%)	⬆️