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   Gravity gradient for a rigid body with small offsets from LVLH. 
   The spacecraft is aligned with LVLH with orbit rate is about the -y axis. 
   This model is valid only for small angles about LVLH.
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   Form:
   [xDot, tGG] = FGravityGradientStiffness( x, t, inertia, orbitRate )
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   ------
   Inputs
   ------
   x          (6,1)  [roll;pitch;yaw;xRate;yRate;zRate]
   t          (1,1)  time
   inertia    (3,3)  Inertia matrix
   orbitRate  (1,1)  Orbit rate

   -------
   Outputs
   -------
   xDot       (6,1)  State derivative
   tGG        (3,1)  Gravity gradient torque

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