run microxs with mpi

[church89]

Description

This PR, together with #3027, extends the use of get_microxs_and_flux to run with MPI.

Checklist

• [x] I have performed a self-review of my own code
• ~[ ] I have run clang-format (version 15) on any C++ source files (if applicable)~
• [x] 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)~