derivative function for pure components
function der_density_phxi extends TILMedia.BaseClasses.PartialGasObjectFunction; input SI.AbsolutePressure p "Pressure"; input SI.SpecificEnthalpy h "Specific enthalpy"; input SI.MassFraction[:] xi "Mass fractions of the first nc-1 components"; input TILMedia.Internals.TILMediaExternalObject gasPointer; input Real der_p "Pressure"; input Real der_h "Specific enthalpy"; input Real[:] der_xi "Mass fractions of the first nc-1 components"; output Real der_d "Density"; end der_density_phxi;