- Fix of mistake in the caloric equation of state of mesitylene and xylene:

The enthalpy of the liquid phase was calculated incorrectly, because the heat capacity had a bug. (Essentially wrong application of Gauss Numerical Integration)

- Fix of wrong transformation between mol / m^3 -> kg/m^3 (molarGasDensity)

Besides that: 
- some cosmetics
- more explanations
- constexpr-calculation instead of hard coded values. Makes the applied equation more traceabel. 
(sorry, comes with the fix)

Reviewed by Benjamin

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9210 2fb0f335-1f38-0410-981e-8018bf24f1b0