RK4:

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   Fourth order Runge-Kutta. Called function is of the form:

   Fun(x,t,varargin)

   Accepts optional arguments that are passed through to Fun. Time is also
   optional.
   This version is streamlined and designed to take advantage of MATLAB's
   function handles feature (MATLAB versions 6 and up only). Passing a 
   string function name (version 5) will also work but is slower.
 
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   Form:
   x = RK4( Fun, x, h, t, varargin )
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   Inputs
   ------
   Fun                Function    Fun(x,{t,...})
   x                  State (column vector)
   h                  Independent variable step
   t                  Current time
   p1...              Optional arguments

   -------
   Outputs
   -------
   x                  Updated state

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