说明：  此函数计算出信号的上下包络，可以用于包络检测，另外通过上下包络计算出信号的均值函数和包络函数，可以用在LMD某个步骤中。
( This function calculates the upper and lower envelope signal of the envelope detection can be used, in addition to the upper and lower envelope by calculating the mean of the function and the signal envelope function can be used in a step of the LMD)