RelativeCoord

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   Computes relative coordinates between two orbits.
   Find relative coordinates between an orbit defined by r and v
   and orbit with position vector r2. This coordinate system uses Kaplan's
   convention and is consistent with LinOrb. 
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   Form:
   d = RelativeCoord( r, v, r2, v2 )
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   Inputs
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   r               (3,:) Position vector
   v               (3,:) Velocity vector
   r2              (3,:) Second orbit
   v2              (3,:) Second orbit

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   Outputs
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   d               (3,:) Relative position [dr;rtheta;z]
                   (6,:) And velocity if v2 is entered

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   References:	 Kaplan, M. H., Modern Spacecraft Dynamics and Control
                J. Wiley, 1986, pp. 109-111.
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