Revised ImageDM-ObsCore architecture

[Douglas Tody]

Hi All -

I made a pass at updating the ImageDM architecture to fully base Image
on the ObservationDM for integration with ObsCore/ObsTAP; in particular
the class structure in UML and related text is completely revised.  This
is updated in the Architecture section of the 10Nov ImageDM draft
(sections 2 - 2.1 of the draft - the rest is unchanged at this point).
Please have a look and comment.  This can be found on the ImageDM TWiki
page:

     http://wiki.ivoa.net/twiki/bin/view/IVOA/ImageDM

A graphic illustrating only the revised ImageDM class structure can be
found here:

     http://wiki.ivoa.net/internal/IVOA/ImageDM/DMArch-ImageFull.png

We have also nearly completed revising the Utypes for ImageDM and
Observation to be consistent with Char2 (Francois and Mireille get the
credit for this).  An updated data model spreadsheet detailing the
revised data model attributes and their Utypes should be available
tomorrow.

This proposal is similar to what Mark suggested earlier for the
SpectralDM to extend Observation, but a bit simpler I think.  In this
version Observation is at the root and Image derives from Observation.
The Dataset element is still present but is integrated directly into
Observation, as Mireille suggested a while back (the Obs attributes move
to Dataset).  Dataset.Datamodel is likewise integrated.  In words, the
core model is Observation, and and ObservationDM instance may have
Dataset attributes.

ObsTAP likewise derives from Observation.  ObsTAP basically specifies
how Observation is mapped onto a static database table, with fixed
column names, coordinate frames, and units; these have to be fixed to
simplify SQL/ADQL queries against the table.  ObsTAP is basically just
Observation (ObsCore) with some restrictions on allowable frames and
units.  ObsTAP also adds an Access element, required for dataset
retrieval, that is not part of the main Observation model.  Aside from
possible issues of differing restrictions on the shared ObservationDM,
ObsCore/ObsTAP and Image (and presumably Spectral) would be compatible
with each other following these changes.

 	- Doug