Add ATO STP2 codes when creating payroll items

All payroll items are required to be given an ATO code to comply with STP2 requirements. It would be useful to have a field to add these ATO codes when setting up or editing payroll items. The added codes would flow through to the payroll system ensuring that every payment had the correct STP2 code when reporting to the ATO. This would remove uncertainty and error when processing wages.