SED data model update

[Douglas Tody]

Jonathan and I have done another iteration on the SED data model, building on the
work done in the past months on Spectrum 1.2 and Photometry.  We can
discuss this in the SED session on Tuesday.  You can find it here:

     http://www.ivoa.net/internal/IVOA/InterOpMay2011SED/SEDDM-20110515.pdf