Updates to mesh vertices and centroids

[pshriwise]

Description

This PR adjusts the vertices and centroid properties on the Python API mesh classes s.t. they always return Cartesian coordinates for any mesh type. Methods for getting vertices and centroid coordinates in other systems have been added for CylindricalMesh and SphericalMesh.

I've also updated the UnstructuredMesh vertices, centroids, and connectivity properties to return the same structure as the other classes with a shape (3, ...) so that they can be unpacked as x, y, z = mesh.centroids when needed.

Checklist

• I have performed a self-review of my own code
  - [ ] I have run clang-format on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
  - [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

I think we hashed a lot of this out offline @eepeterson. In addition to discussing #2720 and #2721, we settled on keeping vertices and centroids as properties with properties for local coordinate systems where applicable (cylindrical and spherical mesh). In the future, we may add a more generic method like get_vertices that would allow an arbitrary transformation to be provided.

FWIW, this is similar to the approach for the Tally.mean property and Tally.get_values method.

[eepeterson]

LGTM!

[shimwell]

Looking forward to making use of this new feature

Would there be any benefit in renaming

cylindrical_vertices to vertices _cylindrical
cylindrical_centroids to centroids_cylindrical
Spherical_vertices to vertices_spherical
Spherical_centroids to centroids_spherical

Then it matches the first word of the strucutred mesh methods and might be more findable with tab complete

Sorry I'm late to comment

[pshriwise]

Then it matches the first word of the strucutred mesh methods and might be more findable with tab complete

Good point! I'll make that update. Thanks @shimwell!