Adding get cylindrical mesh index at specified coordinates

[shimwell]

Description

This PR attempts to add the ability to get the cylindrical mesh index of a x,y,z coordinate. This is useful when users are trying to find the mesh value at a specific location.

usage is ...

openmc.CylindricalMesh(r_grid=(0, 5, 10), z_grid=(0, 5, 10))
r, phi, z = mesh.get_indices_at_coords(1, 0, 1)

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)

[paulromano]

Looks like a nice addition, thanks @shimwell! A few comments below:

[shimwell]

Thanks for the review @paulromano

I have gone through and I believe I've addressed all the review comments and adopted all suggestions.