Don't forget to return self

To retain the functionality where we can chain
function calls to add, link we need to return the
object we are acting on (instead of none).

Change-Id: Iadf0f9d3a21b6d079c1edd29063196180f5222ed

taskflow/patterns/graph_flow.py

@@ -204,12 +204,14 @@ class TargetedFlow(Flow):
         super(TargetedFlow, self).add(*items)
         # reset cached subgraph, in case it was affected
         self._subgraph = None
+        return self
 
     def link(self, u, v):
         """Link existing node u as a runtime dependency of existing node v."""
         super(TargetedFlow, self).link(u, v)
         # reset cached subgraph, in case it was affected
         self._subgraph = None
+        return self
 
     @property
     def graph(self):