Spectrum model update

[Jonathan McDowell]

In honor of the ADASS meeting, I've made updates to
the Spectrum model following comments at the Moscow interop.
I propose that this version be considered a working draft
that is ready to go to PR unless there are substantial 
objections. 
 In the XML schema I have retained the non-array version
of the data, since I think most of those who wanted vectors
are going to use VOTABLE or FITS anyway, and since we can
always later issue a suitable alternative schema without
changing the model itself. 
 The latest version of the model is 
http://hea-www.harvard.edu/~jcm/vo/docs/spec98/spec98d.pdf
 and is referred to currently as V0.98d Rev 4 (dated Oct 13).
Note this version is slightly different from the V0.98d
I released on Oct 9, and incorporates further comments
from R. Thompson, K. McCusker and D. Tody, mostly minor
changes to ensure self-consistency. 
 As I type this, I see that Doug has also sent out a mail
releasing the companion SSA Protocol document at
http://www.aoc.nrao.edu/~dtody/ssa/ssa-v097.pdf

  See some of you in Tucson!
     Jonathan