title: subtract columns in the datatable in pairs
body: <p>I have a data. table</p>
  <pre><code>h1 &lt;- c(rnorm(50, mean = 50, sd = 1),
              rnorm(50, mean = 60, sd = 1),
              rnorm(50, mean = 70, sd = 1),
              rnorm(50, mean = 80, sd = 1))
  
  w1 &lt;- c(rnorm(150, mean = 150, sd = 1),
              rnorm(150, mean = 160, sd = 1),
              rnorm(150, mean = 170, sd = 1),
              rnorm(150, mean = 180, sd = 1))
  
  e1 &lt;- c(rnorm(150, mean = 150, sd = 1),
          rnorm(150, mean = 160, sd = 1),
          rnorm(150, mean = 170, sd = 1),
          rnorm(150, mean = 180, sd = 1))
  
  h2 &lt;- c(rnorm(50, mean = 50, sd = 1),
          rnorm(50, mean = 60, sd = 1),
          rnorm(50, mean = 70, sd = 1),
          rnorm(50, mean = 80, sd = 1))
  
  w2 &lt;- c(rnorm(150, mean = 150, sd = 1),
          rnorm(150, mean = 160, sd = 1),
          rnorm(150, mean = 170, sd = 1),
          rnorm(150, mean = 180, sd = 1))
  
  e2 &lt;- c(rnorm(150, mean = 150, sd = 1),
          rnorm(150, mean = 160, sd = 1),
          rnorm(150, mean = 170, sd = 1),
          rnorm(150, mean = 180, sd = 1))
  
  df &lt;- data.frame(h1,w1,e1,h2,w2,e2)
  
  &gt; df
            h1       w1       e1       h2       w2       e2
  1   49.85148 148.6694 149.4619 49.05355 151.1857 147.7629
  2   49.81708 149.7126 149.1840 50.75627 150.4471 149.2853
  </code></pre>
  <p>I would like to find the difference between the columns in data. table?</p>
  <p>what I want to get:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>h1</th>
  <th>w1</th>
  <th>e1</th>
  <th>h2</th>
  <th>w2</th>
  <th>e2</th>
  <th>h2-h1</th>
  <th>w2-w1</th>
  <th>e2-e1</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>49.85148</td>
  <td>148.6694</td>
  <td>149.4619</td>
  <td>49.05355</td>
  <td>151.1857</td>
  <td>147.7629</td>
  <td>-0,79793</td>
  <td>2,5163</td>
  <td>-1,699</td>
  </tr>
  </tbody>
  </table>
  </div>