中文说明：对一个3*3 的张量进行重定向，使用MATLAB语言编写的PPD（Preservation Of Principal Direction）算法，并且附带测试程序，初次编写，可能有错，希望得到改进意见！
English Description：
To redirect a tensor 3*3, your PPD written using MATLAB language (Preservation Of Principal Direction) algorithms, and testing program, prepared for the first time, and may contain errors, hoping to get suggestions!