Add fusion_neutron_spectrum to openmc.stats module

[paulromano]

Description

This PR adds a new fusion_neutron_spectrum function that returns a Normal distribution representing the neutron energy spectrum from thermonuclear DD or DT fusion reactions in a Maxwellian plasma. The function implements the relativistic interpolation formulas from Ballabio et al. In principle, this could supersede the existing muir function for DD and DT reactions, which uses only the leading-order non-relativistic width formula and neglects the thermal peak shift entirely.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) 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)

[shimwell]

Super to see, I have a made use of a few packages that implement this so it will be most convenient to use it directly in openmc instead.

NeSST is particularly nice as it also has TT reactions, i think it has a slightly more accurate Ballabio formula as well
https://github.com/aidancrilly/NeSST
https://github.com/fusion-energy/fusion_neutron_utils
https://github.com/fusion-energy/openmc-plasma-source

I shall leave this for a day before merging in case others have comments