WIP: Several fixes related to the eigensolvers

Description

There were several problems in the eigensolvers:

• There was an incorrect condition in the CG eigensolver that made the algorithm exit prematurely.
• The subspace diagonalization was never done for the PSD eigensolver.
• The subspace diagonalization was always done twice, except for the RMMDIIS and PSD eigensolvers.
• The PSD eigensolver was not initializing the preconditioner, therefore not using it. A test for the PSD eigensolver was also added.

News snippet

Fixed several bugs in the eigensolvers.

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.