CMToMassPositions:

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   Determine mass positions from a desired center of mass.
   Each mass is assumed to move along a track with a given unit vector.

   Since version 7.
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   Form:
   [rho, kA] = CMToMassPositions( cM, m, d, u )
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   Inputs
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   cM     (3,1)     Requested CM
   m      (1,n)     Mass, including core
   d      (3,n)     Position offset, including core
   u      (3,m)     Unit vector of moving masses

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   Outputs
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   rho    (1,m)     Mass positions along track
   
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