Implement density temperature correction

[darothen]

Currently, the air temperature is diagnosed in both the thermodynamics and RHS routines by using the virtual temperature, $$T_v \equiv T(1 + 0.61w)$$ Rather - at least and most importantly in the RHS evaluation - it should be diagnosed using density temperature, $$T_\rho \equiv T(1 - q_t + q/\epsilon)$$ where $q_t$ is the total vapor/condensate mixing ratio.