Fix the treatment of the Coulomb singularity

Description

The choice of applying the singularity was made based on a wrong condition, and the weight of the singularity was not correct.

Also, the exchange energy is now properly computed for the ACE case, including the singularity energy.

News snippet

Bugfix for Hartree-Fock and full-range hybrids with k-points.

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.