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   Plots statespace data. opt is 'init', 'store', 'plot'

   The calls are

   d = StateSpacePlot( 'init', plots, nPts, nSim )

   plots is a data structure containting
   plots.x       states to plot e.g. [1:4 6 9] or 'all'
   plots.u       inputs to plot or 'all'
   plots.y       outputs to plots or 'all'
   plots.g       state space data structure

   do k = 1:nSim
     d = StateSpacePlot( 'store', x, y, u, d )
   end

   StateSpacePlot( 'plot', d )

-------------------------------------------------------------------------------
   Form:
   d = StateSpacePlot( opt, x, y, u, d )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   opt    (n,:)   Options 'init'
                          'store'
                          'plot'
   x      (:)     State vector "plots" data structure
   y      (:)     Outputs      or nPts
   u      (:)     Inputs       or nSim
   d      (:)     Internal data structure or n steps in the sim
                                  
   -------
   Outputs
   -------
   d      (:)     Internal data structure or list of plots

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