Enumeration defining the reference enthalpy of a medium
type ReferenceEnthalpy = enumeration(ZeroAt0K "The enthalpy is 0 at 0 K (default), if the enthalpy of formation is excluded", ZeroAt25C "The enthalpy is 0 at 25 degC, if the enthalpy of formation is excluded", UserDefined "The user-defined reference enthalpy is used at 293.15 K (25 degC)");