说明：  steppedchirp子函数产生一个子脉冲数为N的步进频信号，子脉冲为chirp信号，参数可在main函数中设置，结果保存在数组y[]中。
(Functions steppedchirp pulses generated for a subset of N step-frequency signal, sub-pulses for chirp signal parameters can be set up in the main function, the result stored in the array y [] in.)