Calculates time derivative of pressure_dT
function pressure_dT_der input Density d "Density"; input Temperature T "Temperature"; input FixedPhase phase = 0 "2 for two-phase, 1 for one-phase, 0 if not known"; input Real der_d "Time derivative of density"; input Real der_T "Time derivative of temperature"; output Real der_p "Time derivative of pressure"; end pressure_dT_der;