change the graph class by a good one

src/jflow/workflow.py

@@ -35,7 +35,7 @@ from inspect import getcallargs
 import jflow
 import jflow.utils as utils
 from jflow.utils import validate_email
-from pygraph.classes.graph import graph
+from pygraph.classes.digraph import digraph
 from jflow.workflows_manager import WorkflowsManager, JFlowConfigReader
 from jflow.utils import get_octet_string_representation, get_nb_octet
 from jflow.parameter import *