VTK: some infrastructure work to support writing of vector fields

- the constructor of VtkMultiWriter now expects a grid view as first
  argument. unfortunately the old API cannot be kept around with this
  change.
- as a follow up beginWrite() does not need the grid view as an
  argument anymore
- if the grid has been changed during two time steps, the new method
  gridChanged() must be called before beginWrite() is called

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6027 2fb0f335-1f38-0410-981e-8018bf24f1b0