Adding statepoint.get_mesh method for finding a mesh

[shimwell]

Description

I would find the ability to get a mesh from a statepoint convenient. Searching for a mesh by ID, name or mesh_type would be helpful.

While making this PR I noticed that the mesh returned from a statepoint does not have a name even if one is set when the mesh is created. I added a TODO for adding a test but I think the bug can be fixed in a separate PR.

This is similar to the get_tally method on the statepoint

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)