title: Multithreaded iteration over groups for Julia GroupedDataFrame
body: <p>I have a <code>GroupedDataFrame</code> in Julia 1.4 (DataFrames 0.22.1). I want to iterate over the groups of rows to compute some statistics. Because there are many groups and the computations are slow, I want to do this multithreaded.</p>
  <p>The code</p>
  <pre><code>grouped_rows = groupby(data, by_index)
  for group in grouped_rows
      # do something with `group`
  end
  </code></pre>
  <p>works, but</p>
  <pre><code>grouped_rows = groupby(data, by_index)
  Threads.@threads for group in grouped_rows
      # do something with `group`
  end
  </code></pre>
  <p>results in <code>MethodError: no method matching firstindex(::GroupedDataFrame{DataFrame})</code>. Is there a way to parallelize the iteration over groups of DataFrame rows?</p>