change the to_graph method to output a directed graph

[HoekR]

The documentation claims that the to_graph method outputs "a networkx directed weighted graph representing the DAFSA", but it now outputs an undirected graph. Changing line 927 in dafsa.py from

graph = nx.Graph()

to

graph = nx.DiGraph()

will remedy this

[tresoldi]

Ops, my fault there! Time for a new release, I suppose, while I slowly work in the new version. Thank you!