RK45:

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

   Fun(x,t,p1,p2,...)

   Accepts optional arguments that are passed through to Fun.
   Time is also optional.

   This function will integrate Fun from the current t to t + hMax.
 
   Since version 1.
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   Form:
   [x, hLast] = RK45( Fun, x, h, hMax, hMin, tol, t, varargin )
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   ------
   Inputs
   ------
   Fun                Function    Fun(x,t,p1,p2...)
   x                  State (column vector)
   h                  Independent variable step
   hMax               Maximum step size
   hMin               Minimum step size
   tol                Tolerance on error
   t                  Current time
   varargin           Optional arguments

   -------
   Outputs
   -------
   x                  Updated state
   hLast              Independent variable step

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	Reference: Cash, J.R., A. H. Karp, "A Variable Order Runge-Kutta Method
              for Initial Value Problems with Rapidly Varying Right-Hand
              Sides," ACM Trans. on Math. Soft., Vol. 16, No.3, Sept.
              1990, pp 201-222.
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