Fix kernel for periodic boundaries

Description

The kernel used only one grid dimension to loop over the periodic points. This, however, fails if more than about 65000 points are in the periodic boundary. Rearrange the grid to split this number over the second and third dimension to avoid this issue.

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.