title: Pandas groupby multiple fields then diff
body: <p>So my dataframe looks like this:</p>
  <pre><code>         date    site country  score
  0  2018-01-01  google      us    100
  1  2018-01-01  google      ch     50
  2  2018-01-02  google      us     70
  3  2018-01-03  google      us     60
  4  2018-01-02  google      ch     10
  5  2018-01-01      fb      us     50
  6  2018-01-02      fb      us     55
  7  2018-01-03      fb      us    100
  8  2018-01-01      fb      es    100
  9  2018-01-02      fb      gb    100
  </code></pre>
  <p>Each <code>site</code> has a different score depending on the <code>country</code>. I'm trying to find the 1/3/5-day difference of <code>score</code>s for each <code>site</code>/<code>country</code> combination.</p>
  <p>Output should be:</p>
  <pre><code>          date    site country  score  diff
  8  2018-01-01      fb      es    100   0.0
  9  2018-01-02      fb      gb    100   0.0
  5  2018-01-01      fb      us     50   0.0
  6  2018-01-02      fb      us     55   5.0
  7  2018-01-03      fb      us    100  45.0
  1  2018-01-01  google      ch     50   0.0
  4  2018-01-02  google      ch     10 -40.0
  0  2018-01-01  google      us    100   0.0
  2  2018-01-02  google      us     70 -30.0
  3  2018-01-03  google      us     60 -10.0
  </code></pre>
  <p>I first tried sorting by <code>site</code>/<code>country</code>/<code>date</code>, then grouping by <code>site</code> and <code>country</code> but I'm not able to wrap my head around getting a difference from a grouped object.</p>