Rank filters

[Johannes Schönberger]

> That's very cool. LIBCVD does something similar in that updates are only
> made to the histogram for pixels that enter or leave the structuring element
> as it's shifted across the image. Any plans to replace erode and dilate with
> rank.minimum and rank.maximum?

 I was planning to add another PR which addresses this. So, I would
leave the current dilate and erode functions and only "redirect"
images of certain dtype to the rank filters, which are only
implemented for uint8 and uint16.