ImageDM "Stable" Version updated

[Douglas Tody]

The "stable" version of the ImageDM has been updated on the ImageDM
TWiki page:  http://wiki.ivoa.net/twiki/bin/view/IVOA/ImageDM

There are actually three new documents, the interim/stable ImageDM
draft, the associated data model spreadsheet giving the details for all
the data model fields, and a spreadsheet from Mireille and Francois
suggesting updated Char2-compliant Utypes for Char (and hence for
ImageDM/Observation/ObsCore).

As a reminder, the point of the "stable" version of ImageDM is to have
something stable to use, consistent with the future Observation /
ObsCore-based data model architecture, synchronized with the SIAV2
draft, for Cube project development beginning this fall.  The idea is to
keep this version of ImageDM stable (few if any changes) while Cube
project development and prototyping goes forward for 4-6 months in
parallel with further development of the ImageDM and related standards.

Eventually things converge and we update our implementations.  It is
clear (especially from the DM list discussions over the past days) that
substantial data model evolution is being considered, and we need some
stability to support development and take-up by the community while
ImageDM and the VO data models and support technology continue to
evolve.

The major change in this version was to the data model classes - the
data model spreadsheet is the best place to review this (the relevant
sections of the document are also updated).  This version is ObsCore
compatible (a superset) with the exception that a few Utypes have been
tweaked as necessary for Char2.  The ObsTAP table field names (these are
what are actually used as the field references in TAP applications) are
unchanged, although new field names have been added for the new fields
of the full model.

I did not attempt to address some open issues such as Observation vs
Dataset vs ObservationDataset and the like; the affected data model
class names (Utypes) are left largely as they were for this version,
while this gets sorted out.   - Doug