Rewrites the long-range term of the stress tensor

Description

Rewrites the long-range term of the pseudopotential contribution to the stress tensor

The new formulation avoids calculation in reciprocal space, and hence properly works in real space. The new formulation seems to converge better with respect to grid spacing, with a smaller prefactor than the prior formulation, and a better power law.

This allows to entirely get rid of the Fourier space for the calculation of the stress tensor.

News snippet

Rewrites the long-range term of the stress tensor for improved efficiency and accuracy.

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.