Enable direct reading of UPF version >= 2 PP files

[abussy]

This PR enables the direct reading of standard pseudopotential UPF files with version >= 2, without the need for the upf_to_json tool. The pugixml library is used for this purpose.

All functions reading information from JSON PP files, namely Atom_type::read_pseudo_uspp and Atom_type::read_pseudo_paw have been overloaded with function reading from xml files (stepwise identical). The relevant function is called depending on the file termination in the SIRIUS input file.

This PR is fully backward compatible. At this stage, the dependency on the pugixml library was made a hard requirement. This would imply a change in the spack recipe for the next release.

Edit: at this stage, it is an open question whether pugixml should be a hard requirement or an option.