Calculates time derivative of density_ph
function density_ph_der input AbsolutePressure p "Pressure"; input SpecificEnthalpy h "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_h "Time derivative of specific enthalpy"; output Real der_d "Time derivative of density"; end density_ph_der;