Remove blank openpyxl sheet. Use standard stream output method.

Two small changes to the xlsx output. First, the Databook output had an extra blank sheet. I changed Databook output function to clear out the worksheets before it adds the ones from the Databook. Second, I changed both the Databook and Dataset output functions to  use the  save_virtual_workbook function from the openpyxl library. This got rid of some extra code and also means the file gets zipped.