This is one step towards turning SurfaceCosseratEnergy into a
generic SurfaceIntegralEnergy class.

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

They were expected to be the factors of a ProductManifold which is
also a template parameter.

Expecting the product and the factors is redundant.  We simply
extract the factors from the product now.

Previously, it expected two factor spaces, as separate template
parameters.  Taking a single ProductManifold type is more consistent
with GeodesicFEAssembler, and it allows to eventually generalize
the code to more factors, if desired.

... 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

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.

With this, the preparatory stretching experiment for film-on-substrate can be done using dune-gfe.
The vertices for adaptive refinement can now be selected without specifying a shell area.

Change the keywords for switching between the solvers to trustRegion and proximalNewton also in film-on-substrate

Use GFE::LocalEnergy, GFE::LocalIntegralEnergy, GFE::SumEnergy (and delete GFE::SumCosseratEnergy).

Virtual Function is deprecated after dune 2.7.

This is where they actually belong, create an own file for identity-dirichlet-values;
before we 'abused' a copy of the file cantilever-dirichlet-values.py

Then each process knows the initial stress free configuration and
can assemble its own part of the grid.