It does not look like that code is used anytime soon, so let's get rid
of it for the sake of tidyness.

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.

Update .gitlab-ci.yml for newer CI images

See merge request !14

This resolves a name ambiguity.

This to clang for pointing out the problem.  I had not noticed it before
because the code was only wrong for dimworld<3, which I never tried.

This makes sequential cosserat-continuum compile again.

OneDGrid does not have method leafbegin (anymore?)

See merge request !6

# Conflicts:
#   dune/gfe/geodesicfefunctionadaptor.hh

These two files are very similar, and merging them will reduce
maintenance effort in the future.  As of this patch, there is
now quite a number of preprocessor directives in the new merged
cosserat-continuum.cc.  That is not pretty, but the new state
is still much easier to maintain than two completely separate
files.  What is more, I hope to be able to reduce some of that
preprocessor trickery by smarter use of the dune-functions
basis trees.

This makes mixed-cosserat-continuum compile again.

This used to work only with dune-fufem bases, but the method
constructBoundaryDofs recently learned to also handle the bases
of dune-functions.

This used to be a yes/no, parameter, now we require 'cube'
rather that 'yes' or 'true'.

But keep compatibility with dune-grid 2.6 as well.

WARNING: These methods implement the projection in quaternion
space.  This is NOT the canonical projection in matrix space.

The original implementation measured the error of SO(3)-valued
functions in quaternion space in a few places.  This patch
converts another occurrence of this to matrix space.

That way, we can use 'if constexpr', and get suitable
writing methods for all kinds of TargetSpace, without
ugly preprocessor tricks.

The current implementation assign one degree of freedom to each
element, regardless of its type.  That only works for quad grids:
We are implementing Lagrange FE here, and these don't assign
degrees of freedom to triangles.

'Only' a stupid typo: '=' instead of '+='.

As of version 2.6, dune-functions doesn't properly do interpolation
of vector-valued functions with scalar-valued bases anymore.