The value dz_max was not correctly computed for the Ewald_long-2D routine.

Description

Bugfix for the calculation of the Ewald summation for 2D periodic systems.

News snippet

Bugfix for the calculation of the Ewald summation for 2D periodic systems.

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.