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   Add mass properties. mB is optional. 
   mA and/or mB may be arrays of data structures.
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   Form:
   m = AddMass( mA, mB, u )
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   ------
   Inputs
   ------
   mA          (:)    Mass data structure
                      .mass    (1,1) Mass
                      .cM      (3,1) Center-of-mass
                      .inertia (3,3) Inertia matrix
   mB          (:)    Mass data structure
                      .mass    (1,1) Mass
                      .cM      (3,1) Center-of-mass
                      .inertia (3,3) Inertia matrix
   u           (1,:)  Units
   
   -------
   Outputs
   -------
   m           (:)    Mass data structure
                      .mass    (1,1) Mass
                      .cM      (3,1) Center-of-mass
                      .inertia (3,3) Inertia matrix

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