Vertex-centric iteration extensions

We would like to make the following extensions to the vertex-centric iterations of Gelly:

• allow vertices to access their in/out degrees and the total number of vertices of the graph, inside the iteration.

• allow choosing the neighborhood type (in/out/all) over which to run the vertex-centric iteration. Now, the model uses the updates of the in-neighbors to calculate state and send messages to out-neighbors. We could add a parameter with value "in/out/all" to the VertexUpdateFunction and MessagingFunction, that would indicate the type of neighborhood.