FIR实现的滤波器，低通带通和高通可选，窗函数也可以自己选，带阻滤波器仅供理解之用，附带有设计的PDF文档-FIR filter implementation, low-pass band-pass and high-pass option, the window function can also choose their own, band stop filter for comprehension purposes, with a design of the PDF document