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   Conmputes the orbit normal from orbital elements.
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   Form:
   n = OrbNEl( el )
     -or-
   n = OrbNEl( i, W )
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   ------
   Inputs
   ------
  One input:
   el              (1,6) Elements vector [a,i,W,w,e,M]
        -or- 

  Two inputs:
   inc             (1,1) Inclination (i)     [rad]
   rA              (1,1) Right ascension (W) [rad]

   -------
   Outputs
   -------
   n               (3,1) Orbit normal

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	References:   Bate, R.R., et. al. Fundamentals of Astrodynamics, p 82.
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