This function returns the numerical derivative of an analytic function. Of special note, is the incorporation of the "complex stepderivative"approach which offers greatly improved derivative accuracy compared to appro

四阶龙格库塔算法。deltaT为积分步长，XX为X_k,X_next为X_k+1，Control以及Param为用户可选参数。微分方程在deriv中定义。-RungeKutta,where deltaT refers to the step size, XX refers to X_k, X_next refers to X_k+1, Control and Param is user defined parameters. The d