New operator map partition function

I would guess that a PartitionMap is used when multiple values should be somehow (in a non-deterministic way) aggregated. Otherwise one would (and should!) use a Map- or FlatMapFunction.

An estimate which is based on the input card is not a good idea, IMHO. 
I would assume that the output card is more likely to be around 1 * DOP, but this is just a gut feeling ;-)