Linear rank filter

So, I'm not sure if this would be appropriate to add to VIGRA or not. This implements a specialized rank filter that operates along single lines of an image. It can work on any type that can be sorted and on images of arbitrary dimension. The algorithm is also designed to work in-place. It does not currently handle multiband images, but this case could be addressed. There are some tests for it and bindings to Python. It requires the container library from boost version 1.56.0 to build.