title: Pandas group by a specific value in any of given columns
body: <p>Given the pandas dataframe as follows:</p>
  <pre><code>   Partner1 Partner2    Interactions
  0  Ann      Alice       1
  1  Alice    Kate        8
  2  Kate     Tony        9
  3  Tony     Ann         2
  </code></pre>
  <p>How can I group by a specific partner, let's say to find the total number of interactions of Ann?</p>
  <p>Something like</p>
  <pre><code>gb = df.groupby(['Partner1'] or ['Partner2']).agg({'Interactions': 'sum'})
  </code></pre>
  <p>and getting the answer:</p>
  <pre><code>Partner Interactions
  Ann     3
  Alice   9
  Kate    17
  Tony    11
  </code></pre>