Symmetrization of the occupation matrices

Description

Symmetrization of the occupation matrices for DFT+U with k-point symmetries.

In order to get good results for DFT+U calculations with k-point symmetries, it is important to symmetrize the density matrix. For spherical harmonics, this is easily done as a rotated spherical harmonics is a linear combination of the spherical harmonics.

The code ow automatically symmetrizes the occupation matrices if k-point symmetries or density symmetrization are activated.

News snippet

Symmetrization of the occupation matrices for DFT+U with k-point symmetries.

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.