Proposal: add 'medoid' to regionprops

[jni]

Description

I've wanted this many times but never had a word for it until someone pointed it out to me recently: the medoid is a representative object from a cluster whose average distance from all the objects in the cluster is minimal (for some definition of "distance"). Unlike the centroid, the medoid has to be an element of the cluster (in our case, segment). So, while centroids can be outside objects if the objects are non-convex (ie many real-world segments), the medoid is guaranteed to fall inside the object. This is useful in a wide variety of settings, such as selecting watershed seed points, or drawing a region adjacency graph.

[VolkerH]

This is related to #5092 , just linking these issues here.