Add random_ray_pincell() example model

[yardasol]

Description

This PR adds a pincell-type model utilizing the same cross section data as random_ray_lattice(). This gives users a smaller building block to create larger models from (i.e. a full core random ray model). This is somewhat redundant given that we can now create source regions from meshes, but I still think it has some use as an illustrative case.

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]

Looks great -- thanks for adding this in! Just a minor idea about naming, but then should be good to go.

[jtramm]

Just a small typo fix, then should be good to go.