code: def axis_angle(self):
          
          qw, qx, qy, qz = self.quaternion
          theta = 2 * np.arccos(qw)
          omega = np.array([1,0,0])
          if theta > 0:
              rx = qx / np.sqrt(1.0 - qw**2)
              ry = qy / np.sqrt(1.0 - qw**2)
              rz = qz / np.sqrt(1.0 - qw**2)
              omega = np.array([rx, ry, rz])
          return theta * omega
docs: :obj:`numpy.ndarray` of float: The axis-angle representation for the rotation.