This simplifies and standardises the handling of radiation in all solvers which
include an energy equation, all of which now support radiation via the
'radiation' fvOption which is selected in the constant/fvOption or
constant/<region>/fvOption file:
radiation
{
type radiation;
libs ("libradiationModels.so");
}
The radiation model, parameters, settings and sub-models are specified in the
'radiationProperties' file as before.