implemented inverse function of vapor pressure to get the temperature at vapor pressure.

Verified as 0.1Mpa: 372.755918611338; 1Mpa: 453.035632391467; 10Mpa: 584.149487998528; reviewed Phil

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8863 2fb0f335-1f38-0410-981e-8018bf24f1b0