Asymmetry of the electric field tensor defined as:
            (V_yy - V_xx)/V_zz

def asymmetry(self):
        """
        Asymmetry of the electric field tensor defined as:
            (V_yy - V_xx)/V_zz
        """
        diags=np.diag(self.principal_axis_system)
        V=sorted(diags, key=np.abs)
        return np.abs((V[1] - V[0]) / V[2])