1p box model: make it use fluid systems

it used to call Fluid::foo() directly. now we do this by means of the
fluid system, so that the same problems for 1p2c can be reused by
1p. Also has the advantage that all box models now provide a fluid
state which makes writing generic code much simpler...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7053 2fb0f335-1f38-0410-981e-8018bf24f1b0