title: Using column name as a new attribute in pandas
body: <p>I have the following data structure</p>
  
  <pre><code>Date         Agric  Food 
  01/01/1990    1.3   0.9  
  01/02/1990    1.2   0.9 
  </code></pre>
  
  <p>I would like to covert it into the format</p>
  
  <pre><code>Date        Sector  Beta
  01/01/1990  Agric   1.3
  01/02/1990  Agric   1.2
  01/01/1990  Food    0.9
  01/02/1990  Food    0.9
  </code></pre>
  
  <p>while I am sure I can do this in a complicated way, is there a way of doing this in a few line of code?</p>