功能：用BFGS算法求解无约束问题：min f(x)

输入：x0是初始点，fun,gfun分别是目标函数及其梯度；

varargin是输入的可变参数变量，简单调用bfgs时可以忽略它

但若其他程序循环调用该程序时将发生重要作用-Function: with BFGS algorithm solving unconstrained problem: min f (x)

　input: x0 is the initial point, fun, gfun respectively are the objective function and its gradient

　varargin is input variable parameter variable, simply call the BFGS can ignore it

　but if other program loop to invoke the program will take place