Add assembleGradient() method to FE assemblers
requested to merge patrick.jaap_at_tu-dresden.de/dune-elasticity:feature/compute-gradient into master
This adds a method to compute only the gradient.
On the pro side it saves a lot of time if only the gradient is needed (for first order methods).
On the con side it introduces some duplicated code (but only for hyberdual numbers).
A test case is added.