PropStateSpace:

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   Propagate a state space model.
   Uses the state space class.
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   Form:
   [x, y] = PropStateSpace( g, x, u )
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   ------
   Inputs
   ------
   g           (:)   Model of type statespace
   x           (:)   State vector
   u           (:)   Input

   -------
   Outputs
   -------
   x           (:)   State vector
   y           (:)   Output vector

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