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 https://github.com/scverse/napari-spatialdata/issues/218.

[codecov[bot]]

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

```diff @@ Coverage Diff @@ ## main #530 +/- ## ======================================= Coverage 92.53% 92.53% ======================================= Files 43 43 Lines 5972 5991 +19 ======================================= + Hits 5526 5544 +18 - Misses 446 447 +1 ``` | [Files](https://app.codecov.io/gh/scverse/spatialdata/pull/530?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse) | Coverage Δ | | |---|---|---| | [src/spatialdata/models/\_\_init\_\_.py](https://app.codecov.io/gh/scverse/spatialdata/pull/530?src=pr&el=tree&filepath=src%2Fspatialdata%2Fmodels%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NwYXRpYWxkYXRhL21vZGVscy9fX2luaXRfXy5weQ==) | `100.00% <ø> (ø)` | | | [src/spatialdata/models/models.py](https://app.codecov.io/gh/scverse/spatialdata/pull/530?src=pr&el=tree&filepath=src%2Fspatialdata%2Fmodels%2Fmodels.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NwYXRpYWxkYXRhL21vZGVscy9tb2RlbHMucHk=) | `87.99% <ø> (ø)` | | | [src/spatialdata/models/\_utils.py](https://app.codecov.io/gh/scverse/spatialdata/pull/530?src=pr&el=tree&filepath=src%2Fspatialdata%2Fmodels%2F_utils.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NwYXRpYWxkYXRhL21vZGVscy9fdXRpbHMucHk=) | `91.60% <94.73%> (+0.53%)` | :arrow_up: |