Returns a string with the structure in XSF format
        See http://www.xcrysden.org/doc/XSF.html

def to_string(self):
        """
        Returns a string with the structure in XSF format
        See http://www.xcrysden.org/doc/XSF.html
        """
        lines = []
        app = lines.append

        app("CRYSTAL")
        app("# Primitive lattice vectors in Angstrom")
        app("PRIMVEC")
        cell = self.structure.lattice.matrix
        for i in range(3):
            app(' %.14f %.14f %.14f' % tuple(cell[i]))

        cart_coords = self.structure.cart_coords
        app("# Cartesian coordinates in Angstrom.")
        app("PRIMCOORD")
        app(" %d 1" % len(cart_coords))

        for a in range(len(cart_coords)):
            sp = "%d" % self.structure.atomic_numbers[a]
            app(sp + ' %20.14f %20.14f %20.14f' % tuple(cart_coords[a]))

        return "\n".join(lines)