RaDec2U:

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   Converts right ascension and declination to unit vectors.

   Since version 1.
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   Form:
   u = RaDec2U( rA, dec )
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   ------
   Inputs
   ------
   rA             (1,n)   Right ascension (rad)
   dec            (1,n)   Declination (rad)

   -------
   Outputs
   -------
   u              (3,n)  Unit vectors

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