String representation of species on the site.

def species_string(self):
        """
        String representation of species on the site.
        """
        if self.is_ordered:
            return list(self.species.keys())[0].__str__()
        else:
            sorted_species = sorted(self.species.keys())
            return ", ".join(["{}:{:.3f}".format(sp, self.species[sp])
                              for sp in sorted_species])