SCH-1073: Add face-centered subset support

[aesgeorges]

SCH-1073: Add face-centered subset support

Summary

Adds support for subsetting face-centered (element) data by computing and assigning z-coordinates at edges and faces, and fixes an issue with sgrid_isel that blocked element-based subsetting.

Changes

• Z-coordinate computation: suxarray/io/_schismgrid.py

  • Added _assign_z_coords to derive 1D static (node_z, edge_z, face_z) and 3D time-varying (node_z_3d, edge_z_3d, face_z_3d) z-coordinates from node-level zCoordinates.
  • Added _calculate_edge_z — averages node z-coordinates for each edge.
  • Added _calculate_face_z — averages node z-coordinates for each face, handling triangular and quad elements.
  • Integrated assignment into _read_schism_grid so z-coordinates are assigned when z-coordinate files are provided.

• Bug fix: suxarray/grid/grid.py

  • sgrid_isel: now excludes inverse_indices along with grid dimensions to avoid invalid xarray-dimension errors during subsetting.

• Tests & fixtures:

  • tests/testfixtures.py: added sxds_element_test fixture loading face-centered test data (out2d_alt_*.nc, zCoordinates_alt_*.nc, verticalVelAtElement_*.nc).
  • tests/test_suxarray.py: added test_subset_elements_bounding_polygon to verify bounding-box subsetting on element-centered data.
  • Added corresponding test data files under tests/testdata/.

Notes

• Edge/face z-coordinate calculations are currently using NumPy; TODOs added to make them dask-compatible for large datasets.
• Face z-coordinate averaging for quad elements uses a simple mean; consider a more rigorous center calculation in a follow-up.

[kjnam]

Alex tested with a full Bay-Delta data and reported the change worked fine.