Noncollinear functionals

Description

First implementation of noncollinear functionals in Octopus. This merge request also contains

• the infrastructures for the noncollinear functionals (not libxc)
• a noncollinear exchange MGGA, derived from the short-range behavior of the noncollinear exchange hole.
• a noncollinear correlation MGGA, from the extansion of the Cole-Salvetti functional to the noncollinear case
• a mixer for vtau, to improve convergence

Tests are added to check for the new functionals within generalized Kohn-Sham and Kohn-Sham using OEP.

News snippet

Initial support of noncollinear functionals.

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.