Changes the identifier of a `TrackerResource`.
The identifier should be a URI or compact URI, but this is not necessarily enforced. Invalid identifiers may cause errors when serializing the resource or trying to insert the results in a database.
If the identifier is set to null, a SPARQL blank node identifier such as `_:123` is assigned to the resource.
| this |
A `TrackerResource` |
| identifier |
a string identifying the resource |