Merge poisson all dims

Description

This MR integrates the screening of the Coulomb interaction, which was implemented in X(poisson1D_solve_direct)(this, pot, rho) into the routine subroutine poisson_solve_direct(this, pot, rho), eliminating the need for the extra routine. The screening is handled as an artificial extra dimension, so no additional branching has to happen in the inner loops.

News snippet

This MR integrates the screening of the Coulomb interaction, which was implemented in X(poisson1D_solve_direct)(this, pot, rho) into the routine subroutine poisson_solve_direct(this, pot, rho).

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.