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.

The code it contains is adolcnamespaceinjections.hh.

I found those wrong types when trying to use a HarmonicEnergyStiffness
with a RigidBodyMotion.

In this case, two different refinements of the grids are not
necessarily geometrically identical.  This means we cannot
use hierarchic search with the global position of a quadrature
point, and expect to find the point on the other grid as well.

Instead, we go done to the coarsest grid, and make the transfer
to the other grid there.

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.

It's the same code, but the old name is deprecated.

Running the test aborts with an error, because the error is exactly
the same as before this patch.