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.

Lisa Julia Nebel authored 5 years ago and Sander, Oliver committed 5 years ago

dune-gfe contains a bit of home-grown infrastructure for distributed
assembly of the tangent stiffness matrix.  This will now be removed
in favour of dune-parmg code, for better maintainability.  Also,
eventually we want the parallel solver of dune-parmg in addition to
the parallel assembly.

The new dependency is optional, to cater for people without interest
in distributed computing.

It is needed for shell problems with non-planar reference configuration.

We start moving to the new infrastructure for functions
and function space bases.

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