Fix xugrid tests in testbank (contourf)

[veenstrajelmer]

Failing testcases:

FAILED tests/test_external_packages.py::test_xugrid_opendataset_ugridplot_contourf - ValueError: triangles are indices into the points and must be in the range 0 <= i < 680 but found value -1
FAILED tests/test_external_packages.py::test_xugrid_opendataset_ugridplot_contour_with_colorbar - ValueError: triangles are indices into the points and must be in the range 0 <= i < 680 but found value -1

Todo:

• contourf fails for face variables with tri/quad mixed topologies xugrid#298
• wait for new release
• update xugrid minimal version in pyproject.toml
• update dfm_tools to accomodate for this new version (see below)
• run testbank: pytest tests/test_dfm_tools.py tests/test_get_nc.py tests/test_xugrid_helpers.py tests/test_external_packages.py

When running the tests with the branch in Deltares/xugrid#299, there are two other testcases that fail:

• test_uda_edges_to_faces_interfaces_to_centers: there seems to be something wrong here for “mesh2d_edge_type”, not for the first var >> postprocess_interpolate_edgevar_tofaces.py maybe dcsm and mwra case are useful for testing >> We need to set fill_value=-1 in dfmt.uda_to_faces()
• test_xarray_decode_default_fillvals: updating the expected value to -1 should do the trick