Feature request: CMFD-like spatial discretization

[tjlaboss]

For geometries other than cylinders, you can't use Cell.setNumRings() to refine your spatial discretization. When comparing results across codes, it is also useful to replicate a mesh tally over one's geometry.

Let's say my geometry looks like this:

Figure 1: Unit cell geometry

And that I want to split it up like so:

Figure 2: 2x2 CMFD mesh

This can currently be done with cmfd.setLatticeStructure(nx, ny).

That's great if you want to set your CMFD mesh to exactly the mesh you use for FSR discretization; but for a fine mesh, that is very slow.

Figure 3: 8x8 mesh

A feature that would be very helpful for me would be a function in the spirit of Cell.setNumRings(n), e.g., Cell.setMeshDivision(nx, ny) -- something that produces the FSRs in Fig. 3 while using the mesh in Fig. 2 for CMFD.

I wonder if this could be implemented by passing a second instance of CMFD to Geometry::segmentize().

[tjlaboss]

Looks similar to #106: "Mask" object

[GiudGiud]

I just added this functionality in my branch.
Let me know how it compares to linear sources for your case

[tjlaboss]

Closed with #387.