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   Sum a set of torques produced by summing T = rxF 
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   Form:
   t = SumRXF( r, f, cM )
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   Inputs
   ------
   r             (3,n) Position vectors
   f             (3,n) Force vectors
   cM            (3,1) Center of mass

   -------
   Outputs
   -------
   t             (3,1) Total torque

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