title: Label the first element in each groupby
body: <p>I have a data frame that looks like the following</p>
  <pre><code>df = pd.DataFrame({'group':[1,1,2,2,2],'time':[1,2,3,4,5],'C':[6,7,8,9,10]})
  </code></pre>
  <pre><code>    group   time    C
  0   1       1       6
  1   1       2       7
  2   2       3       8
  3   2       4       9
  4   2       5       10
  </code></pre>
  <p>and I'm looking to label the first element (in terms of time) in each group as True, i.e.:</p>
  <pre><code>    group   time    C   first_in_group
  0   1       1       6   True
  1   1       2       7   False
  2   2       3       8   True
  3   2       4       9   False
  4   2       5       10  False
  </code></pre>
  <p>I tried several combinations of <code>groupby</code>, <code>first</code> but did not manage to achieve what I wanted.</p>
  <p>Is there an elegant way to do it in Pandas?</p>