2p box model: set the fluid state's temperature in VolumeVariables::updateTemperature

now the only place which stores temperatures is the fluid state which
will help to avoid inconsistencies in the future.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7044 2fb0f335-1f38-0410-981e-8018bf24f1b0