Cosmetics: Added line break.
Renamed potential_and_field to energy_and_forces. Returns energy and forces now. This is needed for the new ChargeOverlap class that now implements (pseudo) nuclear charges. Forces are not simply q*E anymoe, but E (electric field) is really not used anywhere.
Minor optimizations
Cosmetics: Tabs to spaces.
Cosmetics: Tabs to spaces.
Correct signature of method energy_and_forces of coulomb classes.
Added numerical test for electrostatic potential: Electrostatic potential is derivative of energy with respect to charge.