Generates the film/substrate slab combinations for a set of given
        miller indicies

        Args:
            film_millers(array): all miller indices to generate slabs for
                film
            substrate_millers(array): all miller indicies to generate slabs
                for substrate

def generate_surface_vectors(self, film_millers, substrate_millers):
        """
        Generates the film/substrate slab combinations for a set of given
        miller indicies

        Args:
            film_millers(array): all miller indices to generate slabs for
                film
            substrate_millers(array): all miller indicies to generate slabs
                for substrate
        """
        vector_sets = []

        for f in film_millers:
            film_slab = SlabGenerator(self.film, f, 20, 15,
                                      primitive=False).get_slab()
            film_vectors = reduce_vectors(film_slab.lattice.matrix[0],
                                          film_slab.lattice.matrix[1])

            for s in substrate_millers:
                substrate_slab = SlabGenerator(self.substrate, s, 20, 15,
                                               primitive=False).get_slab()
                substrate_vectors = reduce_vectors(
                    substrate_slab.lattice.matrix[0],
                    substrate_slab.lattice.matrix[1])

                vector_sets.append((film_vectors, substrate_vectors, f, s))

        return vector_sets