Fix to translate saturation adjustment to buoyancy in plume

[martinjanssens]

Saturation adjustment did not translate to plume buoyancy, since our function for virtual potential temperature assumed theta had been isolated, while our plume model otherwise assumes we are dealing with theta_l. Have made this dependence explicit by introducing theta_l and diagnosing theta.

A fix that remains to be done (for later) is to also make explicit that the environment should distinguish between theta and theta_l. For now we assume the environment is unsaturated.