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   Computes a gravity gradient torque.
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   Form:
   t = TGG( inr, u, wO )
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   Inputs
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   inr                (3,3) Inertia matrix
   u                  (3,:) Unit vector from the earth to the spacecraft
                            in the spacecraft body frame (the same frame as
                            the inertia matrix.)
   wO                 (1,1) Orbit rate (rad/sec)

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   Outputs
   -------
   t                  (3,:) Torque (units will be consistent with inertia)

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