title: Sum up specific rows for each column
body: <p>I'm sure my problem is easy to solve unfortunately I can't come up with a simple solution for my self. I want to sum up certain rows of a dataset for each column.</p>
  
  <p>My dataset looks like this:</p>
  
  <pre><code>    GIVN  MICP  GFIP
  -2  0.01  0.02  0.01
  -1  0.03 -0.01  0.01
  0  -0.02 -0.03  0.01
  1  -0.04  0.05 -0.02
  2   0.01  0.02  0.03
  </code></pre>
  
  <p>Now I want it to be summed once from row -1 to 1 and from row -2 to 1 for each column.</p>
  
  <p>This should look like this for -1 to 1:</p>
  
  <pre><code>    GIVN  MICP  GFIP
     -0.03  0.01  0.00
  </code></pre>
  
  <p>With the function colSums I only add all rows from each column, which is not what I want to do.</p>