MAJ: Directed graph unable to store multiple parallel edges

[nadicek]

In controller application, DiGraph data structure is used to store the data about the topology. DiGraph is however unable to hold multiple parallel edges (http://networkx.lanl.gov/reference/classes.digraph.html). This can pose a blocking problem in the future.

• source: /ryu/ryu/app/magic.py

[thirjak]

We need to replace DiGraph with MultiDiGraph (http://networkx.lanl.gov/reference/classes.multidigraph.html).