[tei-council] adding (non-)Struct elements to <date> & <time>

[Syd Bauman]

I've removed the <dateStruct> and <timeStruct> elements:
* changed prose of Names & Dates chapter (that's "ND" for those who
  prefer to use a lookup-table :-)
* moved reference to model.datePart from names & dates (ND) to the
  core (ST)
* changed content model of <time> and <date> to 
  ( text | model.gLike | model.phrase | model.datePart | model.global )*
* changed <valDesc> of value= so that it refers to W3C rather than
  ISO spec[1]
* changed other wording in the remarks in the tagdoc
* added <time> to class att.editLike, as <date> is already a member
  of that class
* changed all examples of <(date|time)Struct> to just <date|time>
  (there were more than I expected :-)

While I was there, I also
* corrected the old references to ISO 8601 to be references to W3C
  Schema part 2: datatypes.

Questions/problems
------------------
* Should the zone= attribute of <timeStruct> be retained as a new
  zone= attribute of <time>? My instinct is no, it should not, as
  timezones are expressed as part of the normalized value on value=.

* Should the type= attribute of <dateStruct> and <timeStruct> be
  retained? I'm inclined not to, but some people think everything
  deserves a type=.

* We still need to sort out this precision/exactness stuff.

* We need to think through <dateRange> and <timeRange> with respect
  to data.duration, ISO 8601 range formats, and notBefore= &
  notAfter=.


Note
----
[1] This was one of those prose-spec mismatches that need to be
    caught. This one is probably my fault.