title: Mean per group in a data.frame
body: <p>I have a <code>data.frame</code> and I need to calculate the mean per group (i.e. per <code>Month</code>, below).</p>
  
  <pre><code>Name     Month  Rate1     Rate2
  Aira       1      12        23
  Aira       2      18        73
  Aira       3      19        45
  Ben        1      53        19
  Ben        2      22        87
  Ben        3      19        45
  Cat        1      22        87
  Cat        2      67        43
  Cat        3      45        32
  </code></pre>
  
  <p>My desired output is like below, where the values for <code>Rate1</code> and <code>Rate2</code> are the group means. Please disregard the value, I have made it up for the example.</p>
  
  <pre><code>Name       Rate1       Rate2
  Aira        23.21       12.2
  Ben         45.23       43.9
  Cat         33.22       32.2
  </code></pre>