Support UnstructuredMesh for IndependentSource

[pshriwise]

Description

This PR contains a fix in source.py that allows for application of the UnstructuredMesh class in a MeshSource object. It also has some quality-of-life updates for the UnstrucutredMesh class:

• a decorator for properties of the class that require information loaded from a statepoint file
• addition of the indices property to the UnstructuredMesh class and as an abstract property to the MeshBase class.
• a test that an unstructured mesh class can be applied in a MeshSource

Partially Fixes #2929 -- I'll be creating a follow-on PR to support addition of the statepoint properties from the Python API next.

Checklist

• I have performed a self-review of my own code
• [ ] I have run clang-format (version 15) 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)

[shimwell]

Many thanks Patrick, I can give this go and feedback.

[shimwell]

Just been making use of this branch, it successfully produced a mesh source on an unstructured mesh.

Tagging @rlbarker for your interest

I used this python script, attached .h5m file and produced a nice html plot of the result.
meshsource.zip

[shimwell]

This all looks good @pshriwise and it works nicely over here. The only part I don't quite get is why are the indices a list of tuples instead of a plain old list

[shimwell]

Thanks for explaining that tuple use. All looks good here to me. Shall we leave it open for a few days in case other comments come in.

[shimwell]

We can merge this Friday evening (UK time) unless there are an objections or further comments.

[paulromano]

@pshriwise I made a few little renaming changes -- hope you don't mind! One request that I couldn't handle directly: