Remove homegrown VTKReader, use dune-vtk instead
Simon Praetorius' dune-vtk module has much better support for reading and writing than my old hacks in dune-gfe. Let's start adopting dune-vtk for VTK I/O.
In this initial step, dune-vtk becomes a dependency of dune-gfe, but only an optional one. This may change.