Correct parallel output: pvd files linked to vtu-parts seperated by ":"...

Correct parallel output: pvd files linked to vtu-parts seperated by ":" instead of "-" (the latter is written out by /grid/io/file/vtk/vtkwiter.hh/
=> closing flyspray #169
reviewed by Bernd

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9429 2fb0f335-1f38-0410-981e-8018bf24f1b0