get indices for rectilinear meshes

[marco-de-pietri]

Description

Another little contribution from the OSSFE hackaton for the issue #3867 , it implements the method get_indices_at_coords for the RectilinearMesh class.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[paulromano]

Thanks @marco-de-pietri!