-------------------------------------------------------------------------------
   Computes the right-hand-side of the for a point mass gravity model.
-------------------------------------------------------------------------------
   Form:
   xDot = FOrbKFPointMass( x, t, d )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   x            (6,1)     The state vector [r;v]
   t                      Time
   d            (1,1)     Data structure*
                          .mu                  (1,1)   Gravitational potential
                          .aExt                (3,1)   External acceleration, ex. thruster firings

   -------
   Outputs
   -------
   xDot         (6,1)     The derivative of the state vector

-------------------------------------------------------------------------------