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   Generates the state transition matrix to propagate a quaternion
   one step assuming that the angular velocity is constant. 
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   Form:
   phi = STM( w, dT )
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   Inputs
   ------
   w            (3,1)     Angular rate
   dT           (1,1)     Time change

   -------
   Outputs
   -------
   phi          (4,4)     State transition matrix

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