VTK output: make the used format settable

this reflects Dumux commit r13109, but instead of setting the full
type of the VtkMultiWriter via a property, it is sufficient to set the
VtkOutputFormat property...
VTK output: do not use the pwrite() method of Dune::VTKWriter

because write() does the same as pwrite() with the "path" and
"extendedpath" arguments set to the empty string. Before this patch,
the files which were generated by write() were just overwritten by the
pwrite() method in the parallel case, so this commit can be seen as
just a (relatively) minor performance improvement...