Get the modified structure with the voronoi nodes inserted. The
        species is set as a DummySpecie X.

def get_structure_with_nodes(self):
        """
        Get the modified structure with the voronoi nodes inserted. The
        species is set as a DummySpecie X.
        """
        new_s = Structure.from_sites(self.structure)
        for v in self.vnodes:
            new_s.append("X", v.frac_coords)
        return new_s