With this it will be possible to for the SumCosseratEnergy and the SurfaceCosseratEnergy
    to extend GFE::LocalEnergy, as they need multiple types of TargetSpaces.

Currently, all implementations of variational problems inherit
from LocalGeodesicFEStiffness, which requires gradient and Hessian
implementations.  However, most of these implementations really
only implement the energy, and leave computations of first and
second derivatives to algorithmic differentiation.  To make
this clearer, this patch introduces an abstract base class
LocalEnergy for all implementations that really only have
an energy.