code: public static Workbook createWorkbook(FileFormat format, OutputStream os)
          throws IOException
      {
          return createWorkbook(format, os, null);
      }
docs: Creates a new workbook object.
  @param format The format of the workbook (XLS or XLSX)
  @param os The output stream to write the workbook to
  @return The new workbook created
  @throws IOException if the file cannot be written