RT stellar photon emission rates

This adds a function to compute the stellar photon emission rates needed for RT. The underlying assumption is that the emission rates are intrinsic stellar properties like their age, metallicity, mass etc, and don't depend on the environment of the stellar particle. The longer term plan is to use interpolation tables, so the time data of current time, stellar age, and time step size are necessary.

As per our telecon discussion, the function is called during a drift.

The WIP status is due to this merge request building up on work from !1192 (merged).