自写M函数，做灰度图像的灰度级数及灰度范围的自定义设置。例如：将[0 255]的uint8图像变换成8级灰度的灰度范围为[0 7]图像或者[11.5 25.8]图像。 输出图像为double类，用户可将图像归一化后自行转换为所需类型。-This is a M function written by self, it can be used to rebuild a image with a given number of gray-level and a given gray-scale.