title: change shape of DataFrame for grouping
body: <p>my question is below.</p>
  
  <p>source :</p>
  
  <pre><code>ID  subID_1  value_1   subID_2  value_2  subID_3  value_3
  --  -------  -------  --------  -------  -------  -------
  A    aaa       10       abb       30       acc       40
  B    baa       20       bbb       60       NaN       NaN 
  C    caa       50       NaN       NaN      NaN       NaN 
  </code></pre>
  
  <p>hope : </p>
  
  <pre><code>ID  subID  values  
  --  -----  --------  
  A    aaa     10          
  A    abb     30      
  A    acc     40      
  
  B    baa     20      
  B    bbb     60      
  
  C    caa     50   
  </code></pre>
  
  <p>I tried it using 'unstack()' but result is different comared to what I wanted.<br>
  Please help.<br>
  Thanks in advance</p>