Gray to color image conversion

This function converts a gray image to RGB based on the colors of the source image.
This code was originally inspired by the code gray2rgb by Jeny Rajan and Chandrashekar P.S. The code was optimized and rewritten to more closely achieve what was described in the paper "Transfering Color to Grayscale Images" by Welsh, Ashikhmin and Mueller. Identical results to Rajan's code are achieved much more quickly, especially for large images.