title: Round data to the nearest even integer
body: <p>I think this should be easy to do. I have a data frame <code>df</code> with several columns. In column <code>a</code> I have different values between 1 and 100. </p>
  
  <p>By comparing them I want to create a column <code>new</code> in the <code>df</code> to group them. The function to group them should round the value in <code>a</code> to <strong>the nearest even integer</strong> and save it in <code>df$new</code>. If <code>a = 0</code> it should set <code>new</code> to <code>NA</code>.</p>
  
  <p>See the following example:</p>
  
  <pre><code>... a  ... new
  ---------------
      0      NA
     87.3    88
     88.1    88
     81.7    82
     79.2    80
     89.4    90
  </code></pre>