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   Computes the moment arms for a set of thrusters.
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   Form:
   m = MomArm( r, u, c )
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   Inputs
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   r                (3,:)   Position vectors
   u                (3,:)   Unit vectors in the force direction
   c                (3,1)   Center-of-mass location

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   Outputs
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   m                (3,:)   Moment arms

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