Feature/output vtk path from inputfile

What this MR does / why does DuMux need it:

This adds a path prefix to the name given to the output module. This way the vtk output files can be written to a different folder.

Special notes for your reviewer:

The argument path of the Dune::VTKSequenceWriter only changes the path of the vtu files, not the pvd, so it doesn't give the desired effect.