add flash solver for the immiscible models

it determines pressures + saturations if there are compressible fluid
phases, if not it determines all saturations but leaves the pressures
alone...

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7087 2fb0f335-1f38-0410-981e-8018bf24f1b0