Periodic boundary conditions and PME

Merge of pull requests #34 and #28 

Currently the force computation for electrostatics is missing.
@gmedders DispersionForce long range correction for the 256 molecules case, the `energy_correction` for O-H is -8.2 Kj/mol, this is multiplied by `count` which is 256*512 and becomes a very large number. Do you think there might be a problem here?