Create shuffle and deshuffle vectors

def _generate_permutation(self, npoints):
        """
        Create shuffle and deshuffle vectors
        """
        i = np.arange(0, npoints)
        # permutation
        p = np.random.permutation(npoints)
        ip = np.empty_like(p)
        # inverse permutation
        ip[p[i]] = i
        return p, ip