Merged requested to merge feature/add-vaporpressure-constant-component into master
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.
fixes #1015 (closed)