Fully-relativistic UPF2 pseudopotentials

Description

This merge request adds the support fully-relativistic UPF2 pseudopotentials:

• The parsing of the PP_SPIN_ORBIT block is now done and the corresponding routines are implemented in the upf2 XML parser.
• The code is generalized to support more than one projector per angular momentum
• The XML parser is fixed to load projectors in the right order
• KB code now properly average the projectors is RelativisticCorrection is not set to spin_orbit
• Some small details are fixed
• Some documentation is added

The results were tests for Xe, Bi, and Ta, testing that the eigenvalues given by Octopus for a isolated paramagnetic atom are the same as the one indicated in the pseudopotential files.

The changes also revealed a bug for the PSP8 pseudopotentials with an extension swith of 1, which is now fixed.

News snippet

Adds the support fully-relativistic UPF2 pseudopotentials. Fixes the PSP8 pseudopotentials with an extension swith of 1.

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.