Random Ray FSR Mesh Overlay

[jtramm]

As our random ray solver currently uses the flat source approximation, cells (a.k.a. flat source regions (FSRs)) usually need to be sized at or below the scale of typical mean free paths in order to maintain accuracy. This is presently left to the user to do by manually subdividing cells.

More ideally, we would be able to use some of the abstractions in OpenMC for mesh tallying to overlay a virtual FSR mesh over the grid. This would allow for automatic subdivision of physical cells into smaller FSRs. While the FSR shapes may not be as ideal as if manually divided by a user, in many cases manual subdivision is impractical, so this offers a good path forward in many cases. There are expected to be a few issues that pop up with this (e.g., infinitesimally small FSRs getting generated), but previous work has shown that there are reasonable solutions to these issues and the general method should work.