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   Converts perigee and apogee to semi-major axis and eccentricity
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   Form:
   [a, e] = RPRA2AE( rP, rA )
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   ------
   Inputs
   ------
   rP              (1,1) Perigee
   rA              (1,1) Apogee

   -------
   Outputs
   -------
   a               (1,1) Semi-major axis
   e               (1,1) Eccentricity

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	  References:	  Bate, R.R., et. al. Fundamentals of Astrodynamics, pp. 70-71.
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