Eq2El

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   Converts equinoctial elements to Keplerian elements.
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   Form:
   el = Eq2El( eq )
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   Inputs
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   eq                    Elements vector [a,P1,P2,Q1,Q2,l]

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   Outputs
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   el                    Elements vector [a,i,W,w,e,M]

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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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