Fix Laplacian

Description

Rewriting the 3D Laplacian for the stencil general case, such that it works with all the cells, in particular the bcc cell. A test is added on the bcc cells.

News snippet

Bugfix for the Laplacian in the case of non-orthogonal cells.

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.