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   Computes gravity gradient stiffness from inertia and orbit rate.
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   Form:
   s = GravityGradientStiffness( inertia, orbitRate )
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   Inputs
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   inertia    (3,3)  Inertia matrix
   orbitRate  (1,1)  Orbit rate

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   Outputs
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   s          (2,1)  Stiffness [roll,pitch] (Nm/rad)

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