implicit branch: add attachDofData method to the VTKMultiWriter to uniformly...

implicit branch: add attachDofData method to the VTKMultiWriter to uniformly treat vertex and cell data. Simplify addOutputVtkFields methods of the models correspondingly. Get the global indices from the dofMapper instead of vertexMapper or elementMapper whereever possible, avoiding corresponding if-statements.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/branches/implicit@9792 2fb0f335-1f38-0410-981e-8018bf24f1b0