Geometry cell at volume element center

```discretize_geom()``` only works on axis-aligned, structured, Cartesian meshes. This PR implements a more approximate method that works on all meshes. The method declares whatever cell occupies the center of the mesh volume element to be the representative cell for that mesh volume element. This method is also good for meshes that are guaranteed to be conformal, that is, they contain exactly one cell (so firing rays with discretize_geom would be wasteful).