Add a get_graph_attributes function to the sasa API

The idea is be able to transmit parameters to algo. For that we can take advantage of the graph attributes mechanism of dot.

For instance, one could write dot files that looks like:

 graph g {
  diameter=2
  k=42  

  p1 [algo=p.ml]
  p2 [algo=p.ml]
  p1 -- p2
}

And then access to k and to the diameter as follows:

 let (init_state: int -> 's) =
  fun _ -> 
    let diameter = int_of_string (get_graph_attributes "diameter") in
    let k = int_of_string (get_graph_attributes "k") in
    diameter+ (Random.int k)