This allows CSG cells to be defined in a DAGMC model used for skinning, but it ignores them for skinning (so they will disappear from the model after the first skin operation). This is an attempt to workaround https://github.com/openmc-dev/openmc/issues/2402 to allow us to pre-load the necessary MGXS temperatures into a skinned DAGMC model.
This allows CSG cells to be defined in a DAGMC model used for skinning, but it ignores them for skinning (so they will disappear from the model after the first skin operation). This is an attempt to workaround https://github.com/openmc-dev/openmc/issues/2402 to allow us to pre-load the necessary MGXS temperatures into a skinned DAGMC model.