BigNumber to the
      specified number of significant digits. If rounding is necessary, the
      current <a href='#rounding-mode'><code>ROUNDING_MODE</code></a> setting is
      used.
    </p>
    <p>
      If <code>significant_figures</code> is less than the number of digits
      necessary to represent the integer part of the value in normal notation,
      then exponential notation is used.
    </p>
    <p>
      If <code>significant_figures</code> is omitted, or is <code>null</code> or
      undefined