Add chi cross sections to MGXS machinery

[yardasol]

Description

This PR adds a chi cross section to the existing MGXS machinery. This cross section will be used in the kinetic random ray simulation mode. I have not been able to find anywhere in the MGMC code where chi_prompt or chi_delayed are used, so I have been unable to add chi to the MGMC machinery. If anyone knows where in the source code these cross sections are used, that would be helpful. But if they aren't no problem I suppose.

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)

[nelsonag]

@yardasol, if memory serves, MG transport doesnt use chi, instead it builds the more accurate nu-fission matrix to determine outgoing fission neutron energy from an incident neutron. Does this help clarify?

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