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)