Improve performance for _has_node

- Networkx represents graphs as a dictionary where the nodes are the
  keys. As such, we can use the built-in __contains__ function for fast
  lookup
- We can also iterate the graphs directly without constructing a
  NodeView using self._graph.nodes()

Author: HippocampusGirl

nipype/pipeline/engine/workflows.py

@@ -912,10 +912,12 @@ def _get_all_nodes(self):
         return allnodes
 
     def _has_node(self, wanted_node):
-        for node in self._graph.nodes():
+        if wanted_node in self._graph:
+            return True  # best case scenario
+        for node in self._graph:  # iterate otherwise
             if wanted_node == node:
                 return True
-            if isinstance(node, Workflow):
+            if hasattr(node, "_has_node"):  # hasattr is faster than isinstance
                 if node._has_node(wanted_node):
                     return True
         return False