Provide the length of the flows.

Its useful to be able to check if a flow contains
anything by just inspecting the len() of the flow.
This can be used to avoid running a flow in the
first place if nothing is in said flow.

Change-Id: Ibfea7f10fc67d5dd3ddfa6f6fe6c08805562998a

taskflow/patterns/graph_flow.py

@@ -61,6 +61,9 @@ class Flow(linear_flow.Flow):
                 break
         return runner
 
+    def __len__(self):
+        return len(self._graph)
+
     @decorators.locked
     def add_dependency(self, provider_uuid, requirer_uuid):
         """Connects provider to requirer where provider will now be required

taskflow/patterns/linear_flow.py

@@ -120,6 +120,9 @@ class Flow(base.Flow):
                 except (IndexError, ValueError):
                     pass
 
+    def __len__(self):
+        return len(self._runners)
+
     def _connect(self):
         if self._connected:
             return self._runners