Add a worksheet to the workbook.

def add_worksheet(self):
        """Add a worksheet to the workbook."""
        wsh = self.workbook.add_worksheet()
        if self.vars.fld2col_widths is not None:
            self.set_xlsx_colwidths(wsh, self.vars.fld2col_widths, self.wbfmtobj.get_prt_flds())
        return wsh