Calculates time derivative of density_pT
function density_pT_der input AbsolutePressure p "Pressure"; input Temperature T "Specific Enthalpy"; input FixedPhase phase = 0 "2 for two-phase, 1 for one-phase, 0 if not known"; input Real der_p "Time derivative of pressure"; input Real der_T "Time derivative of temperature"; output Real der_d "Time derivative of density"; end density_pT_der;