Description

Optimization of the Ewald summation for the ion-ion interaction Tke k -> -k symmetry is used for the G-space long-range part, and the short-range part is optimized.

News snippet

Optimization of the Ewald summation for the ion-ion interaction

Checklist

I have checked that my code follows the Octopus coding standards
I have added tests for all the new features added in this request.