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)

In particular, this patch removes tests for the gradient
of the Cosserat energy and related things.  The
CosseratEnergyLocalStiffness class stopped computing
these things back in 2014 (in 63e42ece )
when it was decided that algorithmic differentiation was
the way to go.

With this patch, the test builds again, but it doesn't run yet.
I am afraid that fixing that may require a larger investment
of time...

Only cmake is supported from now on.

It seemed strange to me to demonstrate a second-order tangent field on a
first-order base function.  People may think that this is a necessary restriction.

Also, this patch includes a bugfix:  The basis of the tangent space was hard-wired
to (0,1,0) and (0,0,1).  But this only works if we are at the tangent space at (1,0,0).

In particular, assemblers now hand LocalView objects around, rather than pairs of
elements and LocalFiniteElements.

[cleanup] Move the FD approximation of the GFE function gradient from the GFE function into the test

It is a part of the test harness, and should not clutter the implementation of the
GFE functions themselves.

Use 1e-8: The square root of the machine precision is said to be the best choice.

Needed for projection-based finite elements.

This patch also adds a unit test.  So far this test passes.

The program is not really a test, but it doesn't really belong into 'src', either.