Fix OEP full - spin polarized case

Description

The OEP full was not taking the right occupations for the spin-polarized case.

News snippet

The OEP full was not taking the right occupations for the spin-polarized case.

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.

testsuite/periodic_systems/21-magnon.01-gs.inp

 CalculationMode = gs
 ExperimentalFeatures = yes
 FromScratch = yes
-DisableAccel = yes
 
 ParKPoints = 2
 ParDomains = 2