Writes bus data to an Excel spreadsheet.

def write_bus_data(self, file):
        """ Writes bus data to an Excel spreadsheet.
        """
        bus_sheet = self.book.add_sheet("Buses")

        for i, bus in enumerate(self.case.buses):
            for j, attr in enumerate(BUS_ATTRS):
                bus_sheet.write(i, j, getattr(bus, attr))