Test the implementation of the source terms in the gray radiation
solver.  This does the "relaxation to thermal equilibrium" test as
described in Swesty and Myra (2009) (originally described in Turner
and Stone, 2001).

The gas energy equation as solved in Castro takes the form:

d(rho E)/dt + div . (rho U E + U p) = - kappa_p c (E_r - a T^4)

(see Howell and Greenough, 2003, Eq. 7.  We use the substitution
4 sigma = a c)

Swesty and Myra write e to be gas energy density (in Castro, we
would write this as rho*e, where e is the specific energy).

There is no hydrodynamics, but the matter and radiation are coupled
through the source term.

The problem starts off with a uniform radiation field and density, and
uses a constant opacity.  The initial radiation field is out of
balance with the gas temperature/energy.  An analytic solution for the
radiation energy is provided in Swesty and Myra, eq. 83 and 84.


This is a gray problem, but should also work the same for multigroup
radiation -- since each group couples to the gas energy equation,
the gas energy will see the effect of all of the radiation groups,
and therefore, in the limit of enough groups, should approximate
the gray problem.
