Create and normalize a vector across ring plane.

def prepare_normal_vectors(atomselection):
    """Create and normalize a vector across ring plane."""
    ring_atomselection = [atomselection.coordinates()[a] for a in [0,2,4]]
    vect1 = self.vector(ring_atomselection[0],ring_atomselection[1])
    vect2 = self.vector(ring_atomselection[2],ring_atomselection[0])
    return self.normalize_vector(np.cross(vect1,vect2))