title: Fill NaN in pandas column in both direction
body: <p>In pandas.fillna, </p>
  
  <pre><code>method : {‘backfill’, ‘bfill’, ‘pad’, ‘ffill’, None}, default None
  Method to use for filling holes in reindexed Series pad / ffill: propagate last valid observation forward to next valid backfill / bfill: use NEXT valid observation to fill gap
  </code></pre>
  
  <p>How can I fill values both backward and forward? None of the options seem to do this</p>