mapEdges implemented

Hello!

I tried to implement testSubgraph and mapEdges and I'm looking for some feedback/review.

Some questions:
* Is testSubgraph correct as it is? How exactly is the LocalCollectionOutputFormat used here?
* Why is env.execute() always at the end of tests?
* Why are there tests without any assertions?
* How should testSubgraph be organized? Should we test for filtering vertices and edges in the same function or break it up into two separate functions?

Thanks!