Windowed histogram

Implemented skimage.filter.rank.windowed_histogram, along with required changes to fitler.rank machinery.
We could.....
But the purpose of computing the single coin histogram using np.histogram is so that it is slightly different from the histogram computed by sliding window (which uses a circular structural element), e.g. to highlight that the single coin histogram is 'training data' while the windowed histograms are test data. I think I will modify the comments/docstring to make this more clear.