DisturbanceStruct:

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   Return a default data structure for SailDisturbance.
   Will add the fields to an existing structure d0 if one is input.

   Since version 7.
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   Form:
   d = DisturbanceStruct( d0 )
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   ------
   Inputs
   ------
   d0            (:)    Data structure, optional

   -------
   Outputs
   -------
   d             (:)    Data structure with disturbance fields

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