title: Fill non-consecutive missings with consecutive numbers
body: <p>For a given data frame...</p>
  
  <pre><code>data = pd.DataFrame([[1., 6.5], [1., np.nan],[5, 3], [6.5, 3.], [2, np.nan]])
  </code></pre>
  
  <p>that looks like this...</p>
  
  <pre><code>    0       1
  0   1.0     6.5
  1   1.0     NaN
  2   5.0     3.0
  3   6.5     3.0
  4   2.0     NaN
  </code></pre>
  
  <p>...I want to create a third column where all missings of the second column are replaced with consecutive numbers. So the result should look like this:</p>
  
  <pre><code>    0       1     2
  0   1.0     6.5   NaN
  1   1.0     NaN   1
  2   5.0     3.0   NaN
  3   6.5     3.0   NaN
  4   2.0     NaN   2
  </code></pre>
  
  <p>(my data frame has much more rows, so imagine 70 missings in the second column so that the last number in the 3rd column would be 70)</p>
  
  <p><strong>How can I create the 3rd column?</strong></p>