Multigrid In Energy Preconditioner for Krylov Solvers

We have added a new multigrid in energy (MGE) preconditioner to the Denovo
discrete-ordinates radiation transport code. This preconditioner takes advantage of
a new multilevel parallel decomposition. A multigroup Krylov subspace iterative
solver that is decomposed in energy as well as space-angle forms the backbone of
the transport solves in Denovo. The space-angle-energy decomposition facilitates
scaling to hundreds of thousands of cores. The multigrid in energy precondi-
tioner scales well in the energy dimension and significantly reduces the number
of Krylov iterations required for convergence. This preconditioner is well-suited
for use wi