[WIP] NL-means denoising

Following a message on the mailing-list, here is a first attempt at contributing my non-local means denoising code. It still lacks an example for the gallery. I would also welcome suggestions to improve the speed, since this algorithm is very slow...
oops I think `result` should be initialized as zeros since the central pixel is copied for the (0, 0) shift. Would you agree, @josephsalmon ? (or others :-)