title: SpreadsheetLight get numeric value from cell formula
body: <p>Does anyone know if it possible to retrieve the numeric value of a formula in SpreadsheetLight?  if not is possible to read the formula in OpenXml as numeric?</p>
  
  <p>I need to return 7.07 for cellValue(8,1)</p>
  
  <pre><code>  public static void TestSlWorkbook()
    {
       SLDocument sl = new SLDocument();
       sl.SetCellValue(1, 1, 1.01);
       sl.SetCellValue(2, 1, 1.01);
       sl.SetCellValue(3, 1, 1.01);
       sl.SetCellValue(4, 1, 1.01);
       sl.SetCellValue(5, 1, 1.01);
       sl.SetCellValue(6, 1, 1.01);
       sl.SetCellValue(7, 1, 1.01);
  
       sl.SetCellValue(8, 1, "=Sum(A1:A7)");
  
    }
  </code></pre>