Refactor the code base to remove the PersistentIdentifier  class and just store it as a string in the Resource class. 
            
| 1. | Change lookup to use idenifier property rather than PersistentIdentifier |  | Done | Unassigned | 
| 2. | Remove Persistent Identifier Class |  | Done | Unassigned | 
| 3. | Verify new datasets are correctly listed in the OAI feed |  | Done | Unassigned | 
| 4. | Verify the retracted and deleted datasets are listed in the OAI feed |  | Done | Unassigned |