rank filter activities

@halavaty

I do not remember exactly what we discussed to have as activities for this module: https://git.embl.de/grp-bio-it/image-analysis-training-resources/blob/markdown/modules/filter_rank.md

Having the median filter as an example on a grayscale image makes a lot of sense because one can then use this to then introduce median based local bg subtraction, e.g. in the next module.

But did we have other meaningful ideas?