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   Computes the parameter for any orbit.
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   Form:
   p = Parameter( e, rP )
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   ------
   Inputs
   ------
   e                   Eccentricity
   rP                  Perigee radius

   -------
   Outputs
   -------
   p	                  Parameter

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    References: Bate, R. R., Fundamentals of Astrodynamics, Dover, 1971,
                p. 25.
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