The correct location for zipWithIndex and zipWithUniqueId

[Andra Lungu]

Hey everyone,

We needed to assign unique labels as vertex values in Gelly at some point.
We got a nice suggestion on how to do that in parallel (Implemented in
https://github.com/apache/flink/pull/801#issuecomment-110654447).

Now the question is where should these two functions go? Should they be
part of the API? Something like:

class DataSet<T> {
  public DataSet<Tuple2<Long, T>> zipWithID() {}
}

or should they go in flink-contrib? Fabian, Robert and Till seem to be
in favour of
the second option.

Thanks!

Andra