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.