Use of IVORN for VOEvent unique identifier

[BaptisteCecconi]

In the current model, a VOEvent must have a unique identifier, which is provided in an ivorn attribute.

They have been discussions to use other unique identifiers for this purpose, so this core attribute could be renamed as uri, and the specification should allow the use of an ivo-id or other any type of uri.

Another question should also be discussed: Do we need a persistent identifier for each event? for the stream?