Compute homogenized materials over mesh elements

[paulromano]

Description

This PR adds a new get_homogenized_materials method to the MeshBase class that allows a user to easily get materials that have been homogenized over mesh elements. Right now, my main intended use case for this is mesh-based R2S calculations where the homogenized materials will be activated and lead to decay photon sources.

A few other notes about changes in this PR:

• We had two separate change_directory context manager definitions in the code. I've centralized them in a new utility_funcs.py module and added an easy toggle for getting a temporary directory.
• A check that was added by @shimwell in added check to length of input args for IndependantOperator #2799 was removed because it doesn't work for when multigroup fluxes are passed to IndependentOperator

@pshriwise @eepeterson Can one of you review this?

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)

[eepeterson]

Thanks for the feature @paulromano! All looks good for now.