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   Computes the semi major axis from apogee and perigee radii
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   Form:
   a = RARP2A( rA, rP )
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   Inputs
   ------
   rA             (1,m)  Apogee radius
   rP             (1,n)  Perigee radius

   -------
   Outputs
   -------
   a              (m,n)  Semi major Axis

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