Script for image data reduciton

functions:
main
sort
image_subtraction
image_division
normalize
mean_image

This script is meant to help reduce image data by subtracting out image
data, dividing out image data, finding the mean image, and finding a
normalized image.

How it is applicable:
create a master bias using meadian_image (another script)
subtract the master bias image from all images in a directory
create and average flat field image with mean_image
normalize the average flat field.
then divide the normalized flat field image out of all images in a
directory.