Fluid framework for 2p box model: Use ImmiscibleFluidState instead of TwoPFluidState.

Implement completeFluidState functionality. This does not seem to be the
final solution, since
a) the material law parameters also need to be passed for the
calculation of the capillary pressure.
b) the temperature needs to be set beforehand, since it is no primary
variable.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7042 2fb0f335-1f38-0410-981e-8018bf24f1b0