Add support for revising an order

DrugOrder original = // something
// We wouldn't want to use copy because we don't want to copy fields from BaseOpenmrsData e.g creator. dateCreated etc, also
// fields that are too specific to a single order can't also be copied over e.g. previousOrder, dateStopped etc. 
DrugOrder revised = original.cloneForRevision();
revised.setDosage(100, milligrams);
orderService.saveOrder(revised);

DrugOrder original = // something
DrugOrder revised = new DrugOrder();
revised.setAction(OrderAction.REVISE);
revised.setPreviousOrder(original);
revised.setDosage(100, milligrams);
// copy other properties from original to revised
orderService.saveOrder(revised);