adding RegularMesh.get_indices_at_coords to match CylindricalMesh

[shimwell]

Description

To match the CylindricalMesh which has this function class I am keen to add this get_indices_at_coords function to the RegularMesh class.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• 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)

[GuySten]

I suggest making get_indices_at_coords a required abstract method of StructuredMesh
and implement it also for RectlinearMesh.

[GuySten]

You also should decide on a way to signal that a point is outside the mesh.

[shimwell]

Thanks for this idea @GuySten I have made the abstract method.

Would it be ok if I leave the spherical mesh and rectilinear mesh implementation of get_indices_at_coords to another PR, I added not yet implemented errors for those options if that helps in the short term.

Also the point outside of the mesh question, not sure what is best to do there. Cylindrical mesh doesn't do anything special in that case so this PR is following that precedence. Would a warning message that prints when a user puts in a point outside the mesh be the best option for this?

[GuySten]

I think it is fine.