necessary changes for the modelcoupling ff+pm

set outflow boundary conditions for single equations
function evalPDELab does update of residual with solution vector from
PDELab at the interface


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@4696 2fb0f335-1f38-0410-981e-8018bf24f1b0