REq

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   Computes the orbit radius and the parameter from equinoctial elements.
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   Form:
   [r, p] = REq( 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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   r	               Radius
   p                Parameter

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 References: Battin, R.H., An Introduction to the Methods and Mathematics
             of Astrodynamics, AIAA, New York, 1987, pp. 484-490.
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