[lib] Minimum doc for extract_visitor...

library/cadbiom/models/guard_transitions/extract_visitor.py

@@ -60,6 +60,11 @@ class PathExtractor(object):
     The visitor clean and affect simple node attributes:
     activated: treated - incoming paths explored
     was_activated: reach frontier or not - has meaning if activated = True
+
+    Attributes::
+
+        :param extract_model: The chart model extracted
+        :type extract_model: <ChartModel>
     """
     def __init__(self, model, target, frontier, reporter):
         """