RestrictIDSet:

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   Given an initial set of relative spacecraft IDs, examine the constraints 
   to determine which of these IDs should be included.

   Since version 7.
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   Form:
   relIDs = RestrictIDSet( relIDs, constraints );
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   ------
   Inputs
   ------
   relIDs       (1,:)     Complete relative ID set
   constraints   (.)      Constraints data structure array

   -------
   Outputs
   -------
   relIDs        (.)      Relative ID set to consider

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