Traceback (most recent call last):
  File "pointing_ex.py", line 78, in <module>
    simulate_frame(frame)
  File "pointing_ex.py", line 64, in simulate_frame
    pointing_q = Pointing.axisangle( az, el )
  File "../pylib/pointing.py", line 32, in axisangle
    s = vector.cross( u, v )
AttributeError: 'module' object has no attribute 'cross'