title: Sort pandas dataframe within groups
body: <p>I have a dataframe:</p>
  
  <pre><code>&gt;&gt;&gt; df
       Category  Score
  0        A      1
  1        A      2
  2        A      3
  3        B      5
  4        B      9
  </code></pre>
  
  <p>I expect the output:
  Sorting Score within Category.</p>
  
  <pre><code>&gt;&gt;&gt; df
       Category  Score
  2        A      3
  1        A      2
  0        A      1
  4        B      9
  3        B      5
  </code></pre>
  
  <p>Any ideas?</p>