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

So far, the Cosserat rod energy implementation hat a first-order
finite element space hardcoded.  This patch removes that restriction.
As for the other models in this Dune module, the finite element basis
is now a template parameter of the model energy, and can be set to
any reasonable basis.

This is currently the faster solver

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

Good to measure whether there is locking, and whether the code works
at all.

So far: 2d only

This will be used in the paper with Hanne, Philipp and Markus
on projection-based FE.