Improve the function naming scheme

[danielmwatkins]

Within the preprocessing workflow, there are a handful of functions with names that don't accurately describe what is happening in the function. imsharpen includes image sharpening AND the adaptive histogram step, while imsharpen_gray does not do any sharpening and operates on a grayscale image. normalize_image does not normalize an image, rather it is a morphological operation performing a dilation and reconstruction.