sift算子中高斯金字塔和DOG的生成，只是该算子的最初步骤,sift operator：the generation of Gaussian pyramid and DOG。it is only the first steps

%this code is the Matlab implimentation of David G. Lowe, %"Distinctive image features from scale-invariant keypoints," %International Journal of Computer Vision, 60, 2 (2004), pp. 91-110. %this code should be used only for academic res