nonlinear elimination for radiation diffusion solve

[BenWibking]

For the radiation diffusion or moment equations, after discretizing in time, one can eliminating all variables in the radiation equations except the radiation energy density. After obtaining the radiation energy density from a multigrid solve, the eliminated variables (e.g., material energy) can be recovered independently in each cell via Newton iteration. This could potentially reduce the cost of the radiation solve significantly.

See https://www.osti.gov/pages/biblio/1660523.