Add median_filter using DIPlib

Jens Luchtrequested to merge
add-remove-outliers into master

The DIPlib median filter implementation out performs SciPys ndimage.median_filter by an order of magnitude.

A simple test showed an speed-up in the order of half the CPU cores (or the number of the actual cores, not threads).

See a nice comparison of an elegant implementation for median filters for images

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