title: Pandas: create dataframe with column headers and cell values from tuples in a dictionary
body: <p>I have a simple pandas dataframe, with two columns:</p>
  
  <pre><code>document  document_topics 
  0         [(0, 0.0280), (1, 0.0372), (2, 0.0131), ... (42, 0.0969)]
  1 ...     [(1, 0.0829), (3, 0.0161), (4, 0.0141), ... (27, 0.2275)]
  </code></pre>
  
  <p>The column 'document_topics' is a tuple of (topic, weight). I would like to split 'document_topics' and get a dataframe like:</p>
  
  <pre><code>document  topic_0  topic_1  topic_2 topic_3 topic_4...
  0         0.0280   0.0372   0.0131  NaN     NaN  
  1 ...     NaN      0.0829   NaN     0.0161  0.0141
  </code></pre>
  
  <p>Not every document has all topics associated with it, so I would like to fill those values in with 'NaN'. What is the best way to create this dataframe?</p>