title: add nods and attribute list and KeyError!
body: <p>I have nodes with a list of attributes for each called 'times' in my case.
  I made a simple model like this and I get KeyError'times'. I need my graph save each node with a list of 'times' as an attribute. How can I fix it? </p>
  
  <pre><code>import networkx as nx
  G = nx.DiGraph()
  for u in range(10):
      for t in range(5):
          if G.has_node(u):
              G[u]['times'].append(t)
          else:
              G.add_node(u,times=[t])
  print(G.nodes(data=True))
  </code></pre>