... but disallow a plain pointer.  This increases consistency with the
GeodesicFEAssembler class.

dune-elasticity also has densities, but they implement a slightly
different interface.  Instead of having special code for such
densities in the LocalIntegralAssembler, add a wrapper that
makes densities from dune-elasticity look like dune-gfe densities.

There is no need to wrap the std::function that stores the
Neumann force density into a std::shared_ptr.

Plus some minor cleanup

This replaces the dedicated implementation of the quadrature loop
for harmonic energies -- that is not needed anymore.

In particular this means that the test for it can be removed.
In theory it should be replaced with a test for the harmonic
energy *density*, but that density implementation is so short
that it doesn't really require a separate test.

Rather than using preprocessor switches.  That's too fragile
and not extensible.

If the target manifold is a product space, then ProductManifold must be
used, and not a variadic list of the factor space types.

They do the same thing, essentially.

This is what the non-mixed ADOL-C assembler does, and I want to
merge the two.

The new method is Fufem::markBoundaryPatchDofs, which apparently
works in just the same way.

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.

It was apparently not used.

If you should ever need something like this in the future,
please use periodicbasis.hh from dune-functions.

The old style has been deprecated for a long time, and is not
supported by dune-fufem anymore.

(In particular, the code discretizationerror.hh now requires
functions to satisfy the dune-functions function interface.)

In the wake of this, compute-disc-error.cc gets a number
of changes.  Let's hope I didn't screw up anything.

There are two important changes:  First of all, the new class implements
the `dune-functions` interface rather than the deprecated one based
on inheritance from `VirtualGridViewFunction`. Secondly, the new class
does not hard-wire geodesic interpolation anymore.  Rather, you can
give it interpolation classes which then govern how interpolation is done.

This change was long overdue.  It was now needed to make
gradient-flow.cc compile with the post 2.9 version of dune-fufem again.

The code erroneously used std::tie to combine two local variables
to a single return value.  That may fail because std::tie produces
a tuple of references.  Once the method has returned these references
may or may not be valid anymore.  I am surprised that this bug
has not caused trouble much earlier.

In particular:
- Use the CosseratVTKWriter also for MIXED_SPACE = 1, only print out the deformation though
- Use a SubsamplingVTKWriter only for dim==gridDim==3

Use the grid function provided by the gmsh4-reader as the stressfreegridfunction in nonplanarcosseratshellenergy

This only works for gmsh-files with version 4 or 5, but this is what we want

Add option to define deformation Dirichlet values for each direction separately in cosserat-continuum

Make GFE_ORDER and LFE_ORDER compiler macros and build the necessary combinations of cosserat-continuum-Xd-in-Xd

To build these combinations with the CI-system, install foamgrid for the tests with parmg, vtk, curvedgeometry and curvedgrid