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.