Hotfix magnus

Description

The Magnus propagator was broken in three different ways:

• X(magnus) was broken, adding the wrong terms and (it looks to me) making wrong assumptions about the behavior of hamiltonian_apply.
• The interpolation of the time-dependent terms of the Kohn-Sham potential was broken in 087deac5
• The times at which the interpolation were done were incorrect.

With these changes I now obtain stable propagations that conserve the total energy over long simulations and reasonable time-steps.

Also, the corresponding test is more or less useless. I'll fix this in a different merge request.

News snippet

Fixed Magnus propagator.

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.