Iterates over a TupleStream and buffers Tuples that are equal based on a comparator.
This allows tuples to be grouped by common field(s).
The read() method emits one tuple per group. The fields of the emitted Tuple reflect the first tuple
encountered in the group.
Use the Tuple.getMaps() method to return all the Tuples in the group. This method returns
a list of maps (including the group head), which hold the data for each Tuple in the group.
Note: The ReducerStream requires that it's underlying stream be sorted and partitioned by the same
fields as it's comparator.