原理：以待处理的像素作为中心，取一个nXn的模板，在模板中选择k个与待处理像素的值最接近的像素，将这k个像素的均值替换原来的像素值。

假设n=3，k=5，调用方法：b = KNNMeanFilter(a, 3, 5)-Principle: to be treated as the center pixel, take a nXn template select the template and the pending k-nearest pixel values of pixels, this k pixels mean replacing the original pixel value. Assuming n = 3, k = 5, call the method: b = KNNMeanFilter (a, 3, 5)