This can be used in makeComposedGridFunction to turn quaternion-valued
functions to matrix-type ones, and vice versa.

It computes buckling under a axial compressive displacement load.

The compute-disc-error program reads this to compute discretization
errors.

Doing the data transfer via VTK files would be more convenient,
but that involves a resampling which I'd rather like to avoid.

Previously, the writer requested an FE basis to represent the director
vectors.  With the help of ComposedGridViewFunction this patch allows
to avoid this extra basis, which makes for a much nicer interface
for CosseratVTKWriter.

It is only used by `film-on-substrate.cc` which many people
do no use.

The NonplanarCosseratShellEnergy class contains a reimplementation of
the energy density of CosseratShellDensity.  The code is much easier
to understand and maintain if the density implementation of
CosseratShellDensity is used instead.

... and with constant Lamé parameters.  In many applications
there are constant, and it is convenient to be able to exploit that.

For some reason there were two code paths for the creation of the
solver object: One for dim==dimworld, and one for dim!=dimworld.
These shouldn't really differ, though, and indeed they didn't
as far as I can tell.  Theremore, merge the two code paths.

That is one step towards splitting off the energy density
into a separate class.

Also, it helps to unify the code paths for the different grid dimensions
in cosserat-continuum.cc

And use it in the dim==dimworld part of cosserat-continuum.cc.

For this test to be really helpful it has to use curved grids.

In the code that takes the initial iterate from a while, a call
to 'interpolate' was out-commented, because interpolating an
'EmbeddedGlobalGFEFunction' was not possible back in the days.

That has changed so, and 'interpolate' can now be called.
I still doubt that the code ever worked or works now.
How could it ever be tested?

'#	geändert:       src/cosserat-continuum.cc

This should simplify the code, but it doesn't: Computing the
displacement still needs a lot of code lines.

Just like in cosserat-continuum.cc, there is now a Python class
that implements the initial iterate and the Dirichlet boundary values
at the same time.

This reflects better what the file does.

Rather than hardcoding a straight reference configuration

This is more flexible, and more consistent with what
cosserat-continuum.cc does.

That is simply more flexible than the old parametertree format.

It is required by dune-fufem anyway.

In particular, there is a directory dune/elasticy/densities now.

The ugly preprocessor switch is still there, but I think
the method is more readable now.

The BulkCosseratDensity class implements three types of curvature
tensors. Previously, the preprocessor was used to select the one
to be actually used.  This patches removes the preprocessors checks
and implements run-time switches instead.

Because the part that computes the curvature energy from the
wryness tensor is identical for all curvature tensors that appear
in this implementation.