Dependency on xarray

[dschwoerer]

We are missing a dependency on xarray:

______________ ERROR collecting zoidberg/test_volume_dommaschk.py ______________
ImportError while importing test module '/builddir/build/BUILD/python-zoidberg-0.3.0-build/zoidberg-0.3.0/zoidberg/test_volume_dommaschk.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib64/python3.15/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
zoidberg/test_volume_dommaschk.py:5: in <module>
    import xarray as xr
E   ModuleNotFoundError: No module named 'xarray'. Did you mean: 'array'?
________________ ERROR collecting zoidberg/test_volume_slab.py _________________
ImportError while importing test module '/builddir/build/BUILD/python-zoidberg-0.3.0-build/zoidberg-0.3.0/zoidberg/test_volume_slab.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib64/python3.15/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
zoidberg/test_volume_slab.py:4: in <module>
    import xarray as xr
E   ModuleNotFoundError: No module named 'xarray'. Did you mean: 'array'?
=========================== short test summary info ============================

We should look whether we can drop xarray, as that pulls in a fairly big dependency tree.