- added boxboundaryvariables.hh

- boxfvelementgeometry: BoundaryFace has also i,j now (both defined as the
vertex of the considered scv); this facilitates the implementation of
outflow boundary conditions


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7357 2fb0f335-1f38-0410-981e-8018bf24f1b0