added openmc.Material.mean_free_path function

[shimwell]

This PR proposes that we add a mean_free_path method to the openmc.Material() class.

@jtramm this might be useful when setting the mesh sizes to a couple of mean free paths in your nice FW-CADIS workflow. Where we don't want the weight window mesh voxel sizes to be more than a few mean free paths of the material.

I would be keen to one day extend this to allow photons but I don't think the underlying _calculate_cexs_elem_mat functions support particles other than neutrons.

Example use

import openmc

mat_concrete = openmc.Material(name="concrete")
mat_concrete.add_element("H", 0.168759)
mat_concrete.add_element("C", 0.001416)
mat_concrete.add_element("O", 0.562524)
mat_concrete.add_element("Na", 0.011838)
mat_concrete.add_element("Mg", 0.0014)
mat_concrete.add_element("Al", 0.021354)
mat_concrete.add_element("Si", 0.204115)
mat_concrete.add_element("K", 0.005656)
mat_concrete.add_element("Ca", 0.018674)
mat_concrete.add_element("Fe", 0.00426)
mat_concrete.set_density("g/cm3", 2.3)

print(mat_concrete.mean_free_path(energy=14e6))
>>>8.220341386874415

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)

[jtramm]

LGTM! Thanks @shimwell!