GetStage:

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   Get all of the LV data fields for a particular stage.
   Since version 2015.1
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   Form:
   s = GetStage( lv, k )
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   ------
   Inputs
   ------
   lv        (.)     Launch vehicle data structure
   k         (1,1)   Index of stage to get        

   -------
   Outputs
   -------
   s         (.)     Stage data for stage "k"

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