Light ion transport

[gridley]

Description

This is a relatively complete implementation of light ion transport I wanted to use to explore the possibility of a chain reaction of DT fusion -> 14.1 MeV neutrons -> elastic scattering producing low MeV D's and T's -> more fusion. Turns out the multiplication factor for this both in infinite medium solids and plasmas is like... < 0.01. So it could not be used to build anything workable. It was inspired by this paper.

I am simply opening and then closing this PR since I don't really have the time to make this clean and merge-able, but would like to put the code out there for others to potentially cherry pick from and use. It is by no means complete or full-featured, but in a basic validation test case for a range of proton energies incident on beryllium seems to give answers in solid agreement with MCNP. As a disclaimer this is vibe-coded but with considerable user intervention and a bit of validation.

An example of a shortcoming is that Q<0 reactions might not be handled correctly at the moment (ctrl-f for TODO in the diff here). Another potential shortcoming is that I'm not sure if the angular distributions are correct at all. There are also some extraneous changes in here like an interpretation of k-eigenvalue with DT fusion looking like fission that would be applicable in that aforementioned hypothesized chain reaction, and a global tally for total neutron production per source particle.

The MCNP data was digitized from this recent paper.

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)