BoxDarcyFluxVariables: use fluid state rather than asking the volume

variables. This avoids having to add multi-phase stuff to single-phase
volume variables.

Suggested and reviewed by Philipp.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8734 2fb0f335-1f38-0410-981e-8018bf24f1b0