Store a mesh k-d tree for mesh variable interpolation

[brmather]

Saves having to rebuild a k-d tree every time meshVar.rbf_interpolate is called.

Minor changes

• use np.power instead of np.pow to maintain backward compatibility with numpy < 2.
• modify read_timestep to read mesh vertices from h5f["geometry"]["vertices"] and be compatible with the function call for petsc_save_checkpoint.

[julesghub]

@brmather
Thanks for this - looks great but one tests is failing.
https://github.com/underworldcode/underworld3/actions/runs/15290057983/job/43007729809#step:5:61
please fix.