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:
```dot
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:
```ocaml
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)
```
issue