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   Transforms a vector from the sun sensor frame to the core frame
   when the sun sensor is attached to a solar array that is rotated
   an angle a with respect to the core. When a = 0 the +z array axis
   is aligned with the +z core axis
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   Form:
   v = SA2Core( a, u )
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   ------
   Inputs
   ------
   a            (1,1)     Angle (rad)
   u            (3,1)     Vector in the array frame

   -------
   Outputs
   -------
   v            (3,1)     Vector in the core frame

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