title: Extract the maximum value within each group in a dataframe
body: <p>I have a data frame with a grouping variable ("Gene") and a value variable ("Value"):</p>
  
  <pre><code>Gene   Value
  A      12
  A      10
  B      3
  B      5
  B      6
  C      1
  D      3
  D      4
  </code></pre>
  
  <p>For each level of my grouping variable, I wish to extract the maximum value. The result should thus be a data frame with one row per level of the grouping variable:</p>
  
  <pre><code>Gene   Value
  A      12
  B      6
  C      1
  D      4
  </code></pre>
  
  <p>Could <code>aggregate</code> do the trick?</p>