Disable Credit Note amendment if it has been authorised

It is possible to amend a  CN without having to Undo it first. This means that the API does not trigger an update to the original CN in Xero. However in some cases involving stock item quantity changes, where restocked items have been sold it is not possible to undo the restock so this will also have to be considered. An alternative would be to modify the API triggers to pick up CN updates.