Add sumenergy.hh, this class assembles the a sum of energies for a single element by summing up the energies of each GFE::LocalEnergy.

This class works similarly to the class Dune::Elasticity::SumEnergy, where Dune::Elasticity::SumEnergy extends
Dune::Elasticity::LocalEnergy and Dune::GFE::SumEnergy extends Dune::GFE::LocalEnergy.