Mix and match pseudopotential sets

Implementation of using a PseudopotentialSet in the speciesblock. So now Octopus can do things like this:

%Species
 'C' | species_pseudo | set | hgh_lda
 'O' | species_pseudo | set | pseudodojo_pbe
%

It also uses the default set if it is not specified, so one can change parameters for a default pseudopotential without having to specify the pseudopotential file. For example:

%Species
 'H' | species_pseudo | mass | 2.0
%