[tei-council] datatypes again: make data.key a URI

[Syd Bauman]

I think I'd prefer to leave it a string, but this may require more
thought. My current reasoning is as follows.

If the datatype is xsd:string, it is very easy (both technically and
politically) for someone to restrict it to only URIs.

If the datatype is xsd:anyURI, it is easy technically, but possibly
quite difficult politically to expand it to include something that is
not a URI. (And Sebastian's LDAP example is a good one, as is my key
into my EMS Personnel database, which is not available via a URI.)