title: Filter rows by sum/average of their elements
body: <p>In a R script I have a DataFrame in the format:</p>
  
  <pre><code> V1           V2           V3           V4
  1  HIAT1 3.917271e-05 4.278916e-05 3.793761e-05
  2  SASS6 2.008972e-06 1.890391e-06 2.168946e-06
  3 TRMT13 4.397712e-06 4.724036e-06 4.009512e-06
  </code></pre>
  
  <p>Where the first column is a String name and the following are numeric values. 
  The total number of values is not known a priori, but depends on the dataset provided as input to the script.</p>
  
  <p>I want to extract only the rows for which the sum(or average) of elements from the second to the last is greater than a certain threshold.</p>
  
  <p>How to do that in R-style?</p>