------------------------------------------------------------------------------
   Add mass and mass derivatives using the mass data structure. 
   mB is optional. mA and/or mB may be arrays
   of data structures. This assumes the mass is constant.
-------------------------------------------------------------------------------
   Form:
   [m, mDot] = AddMassDot( mADot, mA, mB, mBDot, u )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   mADot      (:)    Mass dot data structure
                      .mass    (1,1) Mass
                      .cM      (3,1) Center-of-mass
                      .inertia (3,3) Inertia matrix
   mA         (:)    Mass data structure
                      .mass    (1,1) Mass
                      .cM      (3,1) Center-of-mass
                      .inertia (3,3) Inertia matrix
   mBDot      (:)    Mass dot 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