Simple spectral model revision

[Jonathan McDowell]

Dear DM group,

  I have updated the simple SED (Spectral energy distribution) data
model document to reflect comments made in the group and by other
colleagues. The changes are in detail but not in scope, in keeping with
the intention to leave major changes of scope to a later model.

The document, version V0.93, is at
 http://hea-www.harvard.edu/~jcm/vo/docs/spec.html
and is linked to from the IVOA twiki at
 http://www.ivoa.net/twiki/bin/preview/IVOA/IVOADMSpectraWP

  - Jonathan