[implicit] generalize velocity output

Before, velocity output only worked for cubes. This generalizes it to
simplices (box and cc) and prisms/pyramids (box only). Implements
FS#248.

Some reference solutions are adpated due to an improved boundary
condition handling for cell centered.

Reviewed by Alex.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14085 2fb0f335-1f38-0410-981e-8018bf24f1b0