Port Gelly Graph methods to GraphStreams

[balidani]

All relevant Gelly methods should be ported. Some of these are:

• Graph creation
  • fromRandomEdgeStream()
  • fromIncidenceStream()
• Graph stream properties
  • getVertices(), getEdges()
  • getVertexIds(), getEdgeIds()
  • inDegrees(), outDegrees(), getDegrees()
  • numberOfVertices(), numberOfEdges()
• Graph stream transformations
  • mapVertices(), mapEdges()
  • filterOnVertices(), filterOnEdges()
  • subgraph()
  • reverse()
  • undirected()
  • union()
• Neighborhood methods

Each of these have to be considered for both normal streams and (tumbling?) windows.

[vasia]

I'd like to implement some of these to also start getting a bit familiar with the streaming API.
On which branch are you planning to work on these? Shall we create individual issues for each method or just leave a comment here on what we're working on?

[balidani]

Right now I'm working on a test branch called graph-representation or something. Should I move to master? Is there a point of having an interface for GraphStream?

I think we can comment here about the functions we want to implement.

[senorcarbone]

I will work on the merge trees on a different branch probably and merge later ^^

[vasia]

@balidani, now that we have a clearer model in mind, let's move what we have in the master.
And I don't see a point in the interface, since we're going for edge representation only. We could add this later if needed.