The StressFreeStateGridFunction is the actual parametrization from the grid to the
nonplanar Cosserat shell in stress-free state.
The geometries of the StressFreeStateGridFunction will then be used for all calculations
instead of the linear geometries of the piecewise linear grid.

When dune-curvedgeometry is not found, the normal Derivative is set to zero.

Virtual Function is deprecated after dune 2.7.

Several energy density implementations contained code for matrix methods
like sym, skew, transpose etc.  This patch moves them all to the file
linearalgebra.hh, to get rid of the redundancy.

Because it really is a functionality of rotation matrices in quaternion
representation.  Only the Cosserat energies use it at the moment, but
this is coincidental.

In the same process, the method is renamed to quaternionTangentToMatrixTangent.
That seems to be a more telling name.

Previously, they all inherited from LocalGeodesicFEStiffness,
which was weird, because none of them implemented first or
second derivatives as you would expect from something called
'stiffness'.

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.

The one in the Arithmetic namespace does not exist anymore.

Previously, the NonplanarCosseratShellEnergy had the normal field of the unit sphere
hard-wired in the code.  With this patch, using general triangulated surface as
reference geometries is possible.

I want this file with dimworld!=3 grids, even though it only makes sense
to use it if dimworld==3.

With this patch the file still does not compile.  However, the only remaining
problem is the hacky computation of the surface normals, which needs to be
rewritten in full generality soon anyway.

This model was given to me by Patrizio Neff in private communication.

The code is a prototype.  It appears to work, but it has not been tested very
thoroughly, and quite likely there are some bugs left.  Also, the code contains
the shape of the reference configuration hard-wired (a sphere).