1D singularity

Description

Adding the support of the treatment of the Coulomb singularity in 1D. This helps the convergence with respect to k-points of Hartree-Fock calculations.

News snippet

Adding the support of the treatment of the Coulomb singularity in 1D.

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.