Poisson energy

This branch contains my attempt at distilling a standalone PoissonEnergy from Torsten's paper example. @ensslint, @kjako, @reimar, @dpumpe, @parras : If you have time, please have a careful look and let me know what needs to be changed!

I'm especially unsure about the "Phi_h" parameter; most likely this needs a different name and probably some more reworking.