merge from 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. Reviewed by
gruenich.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9881 2fb0f335-1f38-0410-981e-8018bf24f1b0