Implement a new MeshMaterialFilter

[paulromano]

Description

This PR implements a new MeshMaterialFilter that allows tallying specific combinations of mesh elements and materials. In principle, one can already do this by combining a MeshFilter and a MaterialFilter. However, in many cases a single mesh element only contains a few materials, which means tallying every single material for each mesh element would be very wasteful. This new filter works in conjunction with the Mesh.material_volumes() method, which returns a MeshMaterialVolumes object. This filter has a from_volumes() classmethod that will create a filter with bins for all mesh element-material combinations that were found from the call to material_volumes(). So, the workflow is basically:

# Build your model
model = openmc.Model()
...

# Set up a mesh
mesh = openmc.RegularMesh()
...


# Determine materials present in each mesh element
mat_vols = mesh.material_volumes(model, ...)

# Create corresponding filter
mm_filter = openmc.MeshMaterialFilter.from_volumes(mat_vols)

This new filter is one of the last infrastructural pieces needed to carry out mesh-based subvoxel R2S calculations.

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)