[decoupled] generalize velocity output to handle also simplices

So far, velocity output for the decoupled models has been restricted to
cubes. It now also works for simplices such as triangles and
tetrahedrons. For each element, it evaluates the Raviart-Thomas-0
interpolant of the fluxes in the barycenter.

Brought to attention by, discussed with and reviewed by Nicolas.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13610 2fb0f335-1f38-0410-981e-8018bf24f1b0