Currently, densities only get the type used for coordinates
of the integration domain.  However, this is not enough:
The complete domain must be known, and this domain can be either
a grid element or a grid intersection.

As a first small step, this patch introduces ElementOrIntersection
as a template parameter of the CosseratShellDensity class.

Previously, the code would write the reference deformation and the
final orientation.  This patch replaces the reference deformation
by the final deformation.

This is still wrong, because the CosseratVTKWriter expect the
displacement, not the deformation.  That problem is left for later.

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

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.

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.

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

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

Rather than the grid itself.  Because the grid view is all that
a writer needs to know about.

The ProductManifold class generalizes RigidBodyMotion, and can do
everything that the RigidBodyMotion class can.  Therefore there is
no point in keeping RigidBodyMotion any longer.  Having two
implementations for the same thing will just confuse people.

Add a simple test with a grid containing one element testing if the energy is invariant of the number of grid refinements.