All of a sudden it gets confused with a class of the same name
in dune-localfunctions.

It's not clear what it is supposed to be testing at all.

And make it actually test something.

Previously the code only tested whether the AverageDistanceAssembler
computed gradient vector and Hesse matrix.  Now the test actually
tests whether these are correct.

To make it clear that that test does not currently test anything.

I forgot the GridFactory::createGrid did return a plain pointer
as recently as dune-grid 2.6.

This is what GridFactory::createGrid returns anyway.

It is no test.  Rather, it is a program I used to create illustrations
for the paper on GFE test functions (arXiv:1607.07479).  I moved the
program into the git repository of that paper.

This was never a real test.  Rather, it was I toy for me to test
a mathematical property of GFE functions which is known to be
false anyway.

That's already done in adolcnamespaceinjections.hh, in dune-fufem.

This resolves a name ambiguity.

Capturing any polymorphic object by value is wrong,
and gcc 8 started to warn about this in the case
of exceptions.

The latter is deprecated as of dune-functions 2.6.

This makes the test build again.

It also removes the FD test for the gradient of the harmonic energy.
Since the HarmonicEnergyStiffness class does not implement the
gradient anymore, there is nothing to test here.

Otherwise some dune-common stuff will not work properly,
for example copying of FieldMatrices that use adouble
as their number type.

This fixes #1

The formula for the FD gradient never took the distance between
the two Lagrange points into account, even though the gradient
of an interpolation function clearly depends on it.

(cherry picked from commit 7f326ded)