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   Computes the eccentricity from apogee and perigee radii
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   Form:
   e = RARP2E( rA, rP )
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   Inputs
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   rA             (1,m)  Apogee radius
   rP             (1,n)  Perigee radius

   -------
   Outputs
   -------
   e              (m,n)  eccentricity

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