Filter median dtypes

Various edits which don't change the behaviour of the function.
The docstring was misleading since median_filter can handle input images of all dtypes***.  Beginning with the example given in the docstring!
***As long as there are at most 256 different intensity values.  See below.

"In general", applying `median_filter` to arrays of floats, unsigned or negative integers, the output is what you expect, with same type as input.

What users should be warned against is the following: if there are more than 256 different intensity values (in the input image), the filtering cannot be handled properly.

This also raises the question of what sense it makes to "rank" floats...

All this is a bit messy and confusing, but the filter as it is really works so well!!
I guess we will need to come back to it, but I wanted to start with basic improvements.