Add Ewald missing k-vectors
Add Ewald missing k-vectors

The previous implementation only accounted for vectors of the form
(kx, ky, kz) with kx > 0 && ky > 0 && kz > 0; which is wrong ...

This implementation is loosely inspired by LAMMPS, where prefactors are
computed ahead of time to speed-up the whole calculations.

Luthaf/cymbalum,Luthaf/cymbalum