Tutorial on Kohn-Sham inversion and fixes

Description

Tutorial on Kohn-Sham inversion and fixes.

This also revisit the two-particle part of the Kohn-Sham inversion. In particular, in the 1D case, the computation of the Laplacian is replaced by the evaluation of the second derivatives using splines, with a staggered grid in order to force interpolation. This allows for reducing the threshold of what is a small density to a much lower value of 5e-12 with almost no numerical noise.

Closes #924 (closed)

News snippet

Tutorial and improvements on Kohn-Sham inversion in the two-particle case.

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.