RHSOrbitUKF

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   RIght-hand-side for a cartesian orbit.
   t is not used. This is compatible with RK4 and RK45.
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   Form:
   f = RHSUKF( x, t, d )
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   ------
   Inputs
   ------
   x         (6,1)  State [r;v]
   t         (1,1)  Time (not used)
   d         (1,1)  Data structure
                    .a   (3,1) Acceleration
                    .mu  (1,1) Gravitational parameter

   -------
   Outputs
   -------
   f         (6,1)  State derivative

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   References: None
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