add enthalpy, internal energy, heat capacity and vapor pressure for constant component

• check if this changes any tests: I could not find a test with constant component and non-isothermal
• check for consistency with other components: i will adapt simpleh2o accordingly. the napl components still calculate the gas enthalpy as liquid enthalpy + vaporization enthalpy but I am going to leave this as it is.
• add test

fixes #1015 (closed)