实现经典功率谱估计，带详细的解释包含不同函数下BT法，周期图法，Bartlett法和Welch法，只含一个主文件，清晰易懂-Classic power spectral estimation, containing only a master file contains different functions under the BT method, periodogram, Bartlett law and Welch method, with detailed explanations, clear and easy to understand