OEP Slater fast
This commits adds a new implementation of the Slater approximation to the OEP equations, for both collinear and non-collinear spins. This is faster than the previous implementation, which is kept for the moment.
This commit also fixes a bug of accessing the vxc array when the optional argument was not passed.
The Slater approximation is now a stand-alone functional, and not an approximation of the OEP equation, as this is only meaningful for EXX. This is triggered by XCFunctional = slater_x.
A new, faster, implementation of the Slater approximation to the OEP equations, for both collinear and non-collinear spins.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.