title: Shifting a column down by one
body: <p>Say I have a <code>data.frame</code> that looks like this</p>
  
  <pre><code> df &lt;- data.frame(AAA = rep(c(NA,sample(1:10, 1)),5),
    BBB = rep(c(NA,sample(1:10, 1)),5),
    CCC = rep(c(sample(1:10, 1),NA),5))
  
  &gt; df 
     AAA BBB CCC
  1   NA  NA  10
  2    3   7  NA
  3   NA  NA  10
  4    3   7  NA
  5   NA  NA  10
  6    3   7  NA
  7   NA  NA  10
  8    3   7  NA
  9   NA  NA  10
  10   3   7  NA
  </code></pre>
  
  <p>I want to shift column CCC down by one so that all the numbers align in a single row, and then delete the rows that contain no data (often every other row - BUT NOT ALWAYS - the pattern might vary through the <code>data.frame</code>.</p>