Minor fixes

Description

Several minor bugfixes:

• Allow to print the bandgap for fixed occupations. This makes it visible for restarting calculations.
• Allow to compute dipole matrix elements for spinors.
• At the restarting, the contribution from int_dft_u to the energy was not updating, causing weird output for a restart with MaximumIter=0.
• Fix an inconsistency between v_s and the eigenvalues for potential mixing. This was leading to a discrepancy at restarting of a GS calculation.
• Replace an ASSERT by a proper error message.
• Fix a performance issue with the computation of the dipole matrix elements.

News snippet

Minor bugfixes

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.