Bugfix for user-defined and soft-Coulomb potentials in periodic systems

Description

Bugfix: in case of periodic systems, the user-defined and soft-Coulomb potential where not taking into account periodic copies. The result of the revised code has been checked analytically for the two cases.

News snippet

Bugfix: in case of periodic systems, the user-defined and soft-Coulomb potential where not taking into account periodic copies.

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.