title: Find maximum value in a nested dictionary
body: <p>I am having trouble understanding how to pull the maximum value of a nested dictionary when it is structured as so:</p>
  
  <pre><code>dict = {'City': {1: {'avg_dur': 10.58568297387339,
                   'n_trips': 1901,
                   'tot_dur': 20123.383333333313},
               2: {'avg_dur': 12.25947507658035,
                   'n_trips': 2394,
                   'tot_dur': 29349.183333333356},
               3: {'avg_dur': 12.95495652953303,
                   'n_trips': 3719,
                   'tot_dur': 48179.48333333334}}}
  </code></pre>
  
  <p>I am trying to extract the key for the maximum 'avg_trips' function. In the snippet above, I would expect the answer to return 3. I think I need to use lambda here, but I'm not sure how that works with nested dictionaries to this level. </p>