Use disc/sphere footprint in all filters

[haesleinhuepf]

As discovered in #33 our filters (maximum, median, etc) currently use a square-footprint (np.ones), even though a disc/sphere would be biologically more useful. We should update all our filters.

Backwards compatibility

To avoid breaking backwards-compatibility on code-level, we could introduce new functions, e.g. median_filter2, which uses the disc-footprint. The old median_filter function will become deprecated. We add the new function to the menu, and remove the old from the menu. In this way, formerly written workflows produce consistent results, while newly setup workflows will use the new filters. A deprecation-warning can warn users of the old workflows. I also tend to never remove deprecated functions, unless absolutely necessary, aiming for long-term backwards compatibility.