WIP: Catch FloatingPointError exeception in `energy.at` to store crashing position

Most numerical errors (overflows, divisions by zero, …) with NIFTy occur during minimization runs. Tracking down what caused the fault condition is hard, as the NIFTy minimizers only return positions on non-faulty exits and the exact location of the numerical error has to be reconstructed manually by observing the tracelog.

Typically, the numerical errors occur in the energy functionals to be minimized. Because of this, I propose to augment those to pickle their position on internal crashes.

This will help users in reconstructing the state at crash and in finding the crash reasons.