Allow mesh material homogenization to include or exclude voids

[paulromano]

Description

This PR adds an include_void argument to Mesh.get_homogenized_materials. The default is True, which means that homogenization will account for voids (they will effectively increase the volume and decrease the density of homogenized materials). If it is False, voids will be ignored and won't impact the density of the mixture of materials.

To speak to the motivation, I found this to be necessary for a mesh-based R2S workflow where I was only sourcing decay photons in material (using domain rejection to avoid any voids).

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)

[pshriwise]

Looks good! Thanks @paulromano