The test used CosseratEnergyLocalStiffness, which I am in the
process of getting rid of.  And since I eventually want to
get rid of GeodesicFEAssemblerWrapper too, it is not worth the time
to port the test.

... instead of the class CosseratEnergyLocalStiffness.  This is
one step in the quest to get rid of CosseratEnergyLocalStiffness
entirely.

It does not use the main class CosseratEnergyLocalStiffness,
but only the small helper class to access local finite elements
from different types of basis trees.  I am not very happy about
this helper class, but let's be pragmatic and simply copy it
to nonplanarcosseratshellenergy.hh (with a new, unique name).

ADOL-C really only has fmin.  I don't know why this worked before.

The old implementation relied on assignment of FieldMatrix
from scalar, and for some strange reason that stopped working.

* I did not really test a lot of useful stuff.
* The class CosseratLocalEnergyStiffness is deprecated anyway.
* It stopped compiling and I don't know why.

This removes one use of the CosseratEnergyLocalStiffness class,
which is scheduled for complete removal.

If the grid is 2d we know that determinant is equal to
the determinant of the upper-left 2x2 block.

This extracts the density of the Birsan/Neff Cosserat shell model
with planar reference configuration from the CosseratEnergyStiffness
class.

Even though we know that it is always '3'.  Still, using gridDim
whenever that is meant is more expressive than '3'.

That way, we can also set the initial microrotation -- previously
it was hard-wired to the identity matrix.

Being able to explicitly choose the initial microrotation allows
to start, in particular, from a twisted strip.

Rather than keeping a duplicate implementation of the bulk
Cosserat density.

Energies of particular models should not implement external loads
themselves, because that would lead to a lot of code duplication
(and possible inconsistencies).  A one step towards that goal,
allow to construct a CosseratEnergyLocalStiffness object
without external loads.

Previously, the NeumannEnergy class assumed that the grid dimension
was equal to the world dimension.  This is not the case for shells,
where the grid is 2d but the world is 3d.

The present patch generalizes to code to also allow for 2d grids.

Previously, the method basically hand-implemented a writer for
VTK files.  Using dune-vtk instead leads to much shorter and
simpler code, and gives us access to all the dune-vtk niceties,
like binary files and high-order grid cells.

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

That way, a single Python file is enough to describe the entire
boundary value problem.

Rather than as our homegrown .parset.  That way, after more changes
this will allow to have the entire problem description in a single
file, including the boundary data.

Fix derivatives of nonconforming interpolation

See merge request !158

Fix memory access errors when density does not depend on function value

See merge request !157

Instead of with LocalGeodesicFEADOLCStiffness.

The latter should be faster.

If InterpolationDerivatives is told not to compute the derivatives
for the interpolation value, then it sometimes leaves the corresponding
matrix entries uninitialized.  That's okay: they shall not be used
anyway.

However, the code still did use them: It multiplied them by zero.
This is okay if the matrix entries are actual values, but they may
also be NaNs.  In that case, the NaNs propagate, leading to wrong
results.  Fix this by modifying the matrix products to really omit
the uninitialized values.

Let ADOL-C differentiate the energy density and the GFE interpolation separately

See merge request !138

This is faster than using ADOL-C, and does not add much code.

This will allow derived densities to implement the derivatives by hand.
In some cases (e.g. the harmonic energy) these derivatives are so simple
that there is no need to accept the AD overhead for this.

Currently the user code that plugs together the assembler for a
particular problems has to know that ADOL-C is used internally
to compute certain derivatives.  In shows because certain objects
need to be instantiated with adouble as the number type.  One such
example is all objects derived from LocalDensity.

However, if this is to be avoided, then assemblers need to have
a way to turn a (e.g.) 'double' version of a density into an
'adouble' one.  This commit provides this way, by adding a
'makeActiveDensity' method.

Really more of a set of notes, currently.  In particular, it describes
how the element assembler works, which seems to complicated to me for
having only in-code documentation.