non-LTE radiation solve

[BenWibking]

Describe the proposal
For many applications, the radiation emissivity is not a Planck function (i.e., non-LTE).

Often, this also requires solving the level populations of atomic or molecular states to compute the group-averaged opacities. For instance, this is the case for photoionization in HII regions. In this case, the chemistry network should be solved simultaneously with the radiation-matter exchange.

Additional context
The radiation-matter exchange currently assumes i) the emissivity is a Planck function, and ii) the opacities are a function only of temperature and density. The chemistry network currently operates independently from the radiation solve.