Use of the Identifier class is underspecified


In #244, @thomast73 [wrote](https://github.com/FamilySearch/gedcomx/pull/244#issuecomment-16805674):
>    Any notes in the various specializations of Subject seem to really be commentary on the semantic meaning assigned to the use of identifiers of a "known" type in the context of that specialization.

Well, the only such note that I see is on PlaceDescription. It seems to say that an Identifier might point to a "Place Authority" (something like GNIS, I suppose) or to some other PlaceDescription with a type http://gedcomx.com/Primary. I guess the place authority usage might have a type of http://gedcomx.com/Evidence, though the note implies none at all.

All the other uses have been mentioned in passing in Issues, mostly regarding the n-tier implementation. If you leave it up to implementers to do what they want, no two implementations will inter-operate.