Extend level scattering to support incident photons

[GuySten]

Description

This PR extend Level Scattering to support incident photons.
This is a small step in implementing photonuclear physics.

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]

@GuySten Thanks for starting with a small PR to get us toward support for photonuclear physics. This PR just needs some documentation but otherwise it looks good to me.

[paulromano]

This equation should be documented somewhere in our theory/methodology guide (probably docs/source/methods/photon_physics.rst) since the functional form is different than the neutron-nucleus case. You would need to either cite a reference that has this equation or provide a derivation from first principles.

[GuySten]

I've added documentation in docs/source/methods/photon_physics.rst.

[paulromano]

Need to have comments on these data members so someone looking at the class declaration knows what they are.

[GuySten]

I've documented these data members.