Add neumannenergy.hh, it assembles the Neumann energy for a single element on the Neumann Boundary

This class works similarly to the class Dune::Elasticity::NeumannEnergy, where Dune::Elasticity::NeumannEnergy works with
a PowerBasis (for the displacement function) and Dune::GFE::NeumannEnergy works with a CompositeBasis (for the displacement function
and the rotational field) , so the indices to access the different parts of the basis are different.