Add getGroupKey() method to group-at-time operators

[fhueske]

Group-at-a-time operators (Reduce & CoGroup) work on multiple records in one UDF call. Often these UDFs need to access the key that is common to all records of a group.

We could add a function to set a the key of a group before the UDF is called (setGroupKey()) and a function to get the key (getGroupKey()) that can be called from the UDF.

What do you think about this?

[aalexandrov]

idea. Should I take a look?

[fhueske]

😄 Please do so!