函数逼近的MATLAB程序，本程序设计一个两层的bp网络用于函数逼近，隐层的激活函数为 tansig,输出层激活函数为purelin线性函数

-Function approximation of the MATLAB program, the program design of a two-tier network for function approximation bp, hidden layer activation function is tansig, the output layer activation function is a linear function purelin