adjust to xarray datatree

[melonora]

closes #735
closes #740

Xarray has archived the datatree repository and includes DataTree now directly in xarray. This comes with some changes, most notably the spatialdata repo is affected by the fact that DataTree.from_dict now requires a xr.Dataset instead of xr.DataArray, with the latter being marked as an inappropriate value. This PR fixes that.

Note that the PR in spatial-image/multiscale-spatial-image#94 is still open for now so I did not add a new flag yet in pyproject.toml. This would have to be done prior to merging this PR.

Tests have been performed with the branch of multiscale-spatial-image in the mentioned PR and all pass locally.

[melonora]

I have added a decorator as now with any function mapping over datasets in the DataTree we run into the problem of the root level not containing anything and due to that functions fail. An example is transpose. This method is not directly available anymore on the DataTree level. You would basically have to do datatree.map_over_datasets but this fails because the root level does not have dims (it is an empty set). Given that I could see the usefulness across the repo I added the decorator in this PR so it can be imported in other repos as well.

[codecov]

Codecov Report

Attention: Patch coverage is 98.76543% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.85%. Comparing base (e27dbdd) to head (71531ce).
Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata/_core/operations/vectorize.py	75.00%	1 Missing ⚠️
src/spatialdata/_types.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #752      +/-   ##
==========================================
- Coverage   91.89%   91.85%   -0.05%     
==========================================
  Files          45       45              
  Lines        6919     6885      -34     
==========================================
- Hits         6358     6324      -34     
  Misses        561      561              

Files with missing lines	Coverage Δ
src/spatialdata/_core/_deepcopy.py	98.36% <100.00%> (-0.03%)	⬇️
src/spatialdata/_core/centroids.py	100.00% <100.00%> (ø)
src/spatialdata/_core/concatenate.py	91.59% <100.00%> (ø)
src/spatialdata/_core/data_extent.py	97.91% <100.00%> (-0.02%)	⬇️
src/spatialdata/_core/operations/_utils.py	92.15% <100.00%> (-0.16%)	⬇️
src/spatialdata/_core/operations/aggregate.py	94.35% <100.00%> (-0.04%)	⬇️
src/spatialdata/_core/operations/map.py	97.77% <100.00%> (-0.05%)	⬇️
src/spatialdata/_core/operations/rasterize.py	90.45% <100.00%> (-0.04%)	⬇️
src/spatialdata/_core/operations/rasterize_bins.py	89.88% <100.00%> (ø)
src/spatialdata/_core/operations/transform.py	90.82% <100.00%> (-0.04%)	⬇️
... and 25 more