Extend Kalbach Mann systematics to support incident photons

[GuySten]

Description

This PR extend Kalbach Mann systematics to support incident photons.
This is another small step in implementing photonuclear physics.

A roadmap for the photonuclear physics feature can be found here #1941.

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)

[GuySten]

Thanks for reviewing, @shimwell.
I've answered all your concerns.