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   Converts conventional elements to  equinoctial elements.
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   Form:
   eq = El2Eq( el )
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   ------
   Inputs
   ------
   el                    Elements vector [a,i,W,w,e,M]

   -------
   Outputs
   -------
   eq                    Elements vector [a,P1,P2,Q1,Q2,l]

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   References: Battin, R.H., An Introduction to the Methods and Mathematics
               of Astrodynamics, AIAA, New York, 1987, pp. 490-492.
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