Add suggestion from code review for add_nodes

Author: HippocampusGirl

nipype/pipeline/engine/workflows.py

@@ -333,7 +333,7 @@ def add_nodes(self, nodes):
         newnodes = []
         all_nodes = self._get_all_nodes()
         for node in nodes:
-            if self._has_node(node):
+            if node in all_nodes:
                 raise IOError("Node %s already exists in the workflow" % node)
             if isinstance(node, Workflow):
                 for subnode in node._get_all_nodes():