title: Sorting by absolute value without changing the data
body: <p>I'm looking for a simple way to sort a pandas dataframe by the absolute value of a particular column, but without actually changing the values within the dataframe. Something similar to <code>sorted(df, key=abs)</code>. So if I had a dataframe like:</p>
  
  <pre><code>    a   b
  0   1   -3
  1   2   5 
  2   3   -1
  3   4   2
  4   5   -9
  </code></pre>
  
  <p>The resultant sorted data when sorting on 'b' would look like:</p>
  
  <pre><code>    a   b
  2   3   -1
  3   4   2
  0   1   -3
  1   2   5 
  4   5   -9
  </code></pre>