This patch contains only whitespace changes.

Previously, some code was in Dune::GFE, some was in Dune::,
and a lot of it was in the global namespace.

I wrote them years ago mainly because I thought they should exist.
But they were never actually used them for anything, and I don't
really know were they could be used either.  Supposedly they
could be useful for implementing problems without a minimization
formulation, but that's not currently planned, and I don't know
what the precise requirements would be, either.

Since the code is short and almost trivial, and easily resurrected,
this commit removes the two files.  For newcomers this will mean
two less files to wonder about.

There are two important changes:  First of all, the new class implements
the `dune-functions` interface rather than the deprecated one based
on inheritance from `VirtualGridViewFunction`. Secondly, the new class
does not hard-wire geodesic interpolation anymore.  Rather, you can
give it interpolation classes which then govern how interpolation is done.

This change was long overdue.  It was now needed to make
gradient-flow.cc compile with the post 2.9 version of dune-fufem again.