  Note that the three files that define the quadrature rule
      are assumed to have related names, of the form
      <ul>
        <li>
          <i>prefix</i>_<b>x.txt</b>
        </li>
        <li>
          <i>prefix</i>_<b>w.txt</b>
        </li>
        <li>
          <i>prefix</i>_<b>r.txt</b>
        </li>
      </ul>
      When running the program, the user only enters the common <i>prefix</i> 
      part of the file names, which is enough information for the program
      to find all three files.
