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   Computes planet centered orbital elements. 
   If no outputs are specified it will plot the planet centered orbit.
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   Form:
   el = PtchCnc( vC, vPC, rC, rPC, muC, muP )
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   Inputs
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   vC		      (3,1)   Velocity of the spacecraft in the central frame
   vPC         (3,1)   Velocity of the planet in the central frame
   rC          (3,1)   Position of the spacecraft in the central frame
   rPC         (3,1)   Position of the planet in the central frame
   muC         (1,1)   Gravitational parameter of the central body
   muP         (1,1)   Gravitational parameter of the planet

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   Outputs
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   el          (:)     Elements in the planet frame
                       el.a, el.e, el.i, el.w, el.L, el.M
             
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