fvOption::radiation: New fvOption providing the radiation source to the energy equation

Radiative heat transfer may now be added to any solver in which an energy
equation is solved at run-time rather than having to change the solver code.

For example, radiative heat transfer is now enabled in the SandiaD_LTS
reactingFoam tutorial by providing a constant/fvOptions file containing

radiation
{
    type            radiation;
    libs ("libradiationModels.so");
}

and appropriate settings in the constant/radiationProperties file.