Enable the LegendreFilter filter to be used in photon tallies for orders greater than P0.

[nelsonag]

Description

As noted in #3243 , the LegendreFilter does not work for photon scores. The change was quite simple as a local variable, mu, was used for the change-in-angle during scattering kinematics and never stored on p.mu. This PR corrects this. Results can be seen in the attached .zip which contains the model.xml, the original tallies.out, and a new tallies.out created from the code in this PR. Note the model and original tallies.out is the same as in #3242.
photon_mu.zip

Note, this PR is baselined off of #3242 and that should be merged first.

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)

[paulromano]

Thanks @nelsonag!