Support cell densities in the random ray solver

[nuclearkevin]

Description

This PR adds support for distributed cell densities (added in #3546) in the random ray solver to enable density feedback for multiphysics applications. A cell density multiplier is tracked in each source region (ratio of the cell density to the material density), and used to correct the macroscopic cross section where required. Two new tests are added for this capability, an eigenvalue calculation (to ensure all quantities are tallied correctly) and a fixed source calculation (to ensure the division of the external source by the total cross section is performed correctly). The gold files are generated by manually scaling the cross sections by the ratio of the cell density to the material density.

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)

[jtramm]

This looks great @nuclearkevin -- thanks for submitting this PR! Just a few small questions/comments for your consideration, but otherwise this is in good shape.

[jtramm]

Looks good -- thanks for updating the test!