groupEdges() now compiles. Still need some unit tests
Added a groupEdges and a groupEdgeTriplets method. For some reason the groupEdgeTriplets method isn't properly iterating through the set of edges and thus is returning the wrong result. groupEdges seems to be working.
Fixed groupEdgeTriplets - it now passes a basic unit test.

The problem was with the way the EdgeTripletRDD iterator worked. Calling
toList on it returned the last value repeatedly. Fixed by overriding
toList in the iterator.