title: How should I fill na using the fillna result of the previous row?
body: <p>Suppose I have the following dataframe...</p>
  
  <pre><code>money change
  2     2
  1     1 
  na    2
  na    2
  na    1
  1     1
  </code></pre>
  
  <p>I would like the na value be the previous value plus the change value from the previous row. Note that non-nan entries do not necessarily follow this pattern. 
  How should I fill the na entries under column money so that it looks like the following...</p>
  
  <pre><code>money change
  2     2
  1     1 
  2     2
  4     2
  6     1
  1     1
  </code></pre>