title: Pick the nth element of every group in a grouped dataframe
body: <p>Suppose I create the following dataframe</p>
  <pre><code>using DataFrames
  df = DataFrame(A = rand(500), B = repeat(1:10, inner=50), C = 1:500)
  </code></pre>
  <p>and I can do a <code>groupby</code>:</p>
  <pre><code>grouped_df = groupby(df,&quot;B&quot;)
  </code></pre>
  <p>I will end up with 10 groups. How can I choose, say, the third element of each group, and combine them into a new dataframe? That is, I would like a new dataframe of 10 rows, with each row being the third element of each of the groups?</p>
  <p>I looked into <code>combine</code>, but couldn't find a solution. Can I get a hint?</p>