Oep general fixes spin polarized and mixing scheme
This merge request
- fixes the spin polarized OEP routine.
- adds a new mixing mode for the OEP (Barzilai-Borwein (BB)) Reference: T. W. Hollins, S. J. Clark, K. Refson, and N. I. Gidopoulos, !%Phys. Rev. B 85<\b>, 235126 (2012)
- prints the convergence of the OEP potential to the static/convergence file
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.