Use plain std::vector<double> for element energy gradients

This is what dune-function mandates. We don't actually use dune-functions here a lot, but it is still nice to stay in the spirit. Also, I am thinking about using dune-functions bases for some indexing work in the future.

Plus, I hope that getting rid of the blocking will improve run-times of debug builds a little bit, in particular in the LocalIntegralStiffness class.