Calculates time derivative of density_ps
function density_ps_der input AbsolutePressure p "Pressure"; input SpecificEntropy s "Specific Entropy"; 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_s "Time derivative of specific entropy"; output Real der_d "Time derivative of density"; end density_ps_der;