Helmholtz Decomposition surface correction

Description

This merge request added the logic for computing the surface correction for the Helmholtz decomposition. In particular, it is now possible to compute the surface points for the parallelepiped, sphere, and cylinder box. These points are then used to compute the surface correction for Helmholtz. However, the surface correction is currently disabled because it is not possible to run it in parallel.

The module helmholtz_decomposition was re-engineered: now it is possible to use interfaces to call the subroutines, thus making the code cleaner.

The unit tests for the Helmholtz decomposition were updated:

• A module dedicated to the test was created
• The Gaussian field test is now in Coulomb Gauge, which leads to meaningful results

Follow ups

Check Issue 705 #705

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.