This code (with exceptions) really belongs into the dune-elasticity module.  It was in dune-gfe
only for historical reasons.  This patch brings a driver program finite-strain-elasticity,
a simple trust-region solver, and several new hyperelastic energies.

Unfortunately, this patch is far from perfect:
- It does not use Jonny's framework, but duplicates a lot of things.
  In particular my code that interfaces with adol-c exists in similar
  form already.  The two should be merged eventually.  Apologies.
- Same for the material definitions
- The trust region solver should be in dune-solvers.  However, it currently
  calls the FE assemblers, which should not be done within dune-solvers.

I intend to address these issues eventually.  However, right now I need to do
at least the first step and get the stuff out of dune-gfe.

This is the first code in this module that uses the new dune-functions module.