Updated ImageDM and SIAV2 drafts

[Douglas Tody]

The Image data model (ImageDM) and associated image access protocol
(SIAV2) documents have been updated on the IVOA TWiki (updated versions
were actually uploaded several weeks ago to support author group
discussions, but were not announced at the time).

The ImageDM doc was largely written from scratch and replaces the
placeholder version from the May interop.  The SIAV2 draft has also been
largely rewritten to make use of the closely linked ImageDM and other
recent DAL developments such as DALI.  Full support is provided for
multidimensional image data including large data cubes and sparse cubes.

The drafts live on these pages:

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

Neither document is complete yet, although the ImageDM draft comes close
so far as the core model is concerned.  The main thing to be updated at
this point is the data access model for slicing and dicing, computing
moments,etc., of large image cubes.  We hope to update one or both
documents prior to the interop to reflect author group discussions and
prototyping over the past month or so.

These drafts are also the basis for the cube project data access (SIAV2)
prototype being developed by VAO, as Ray mentioned on the list a few
days ago.  The VAO SIAV2 prototype includes test multidimensional image
datasets for ALMA, JWST, JVLA (and legacy VLA), JCMT, Keck (Osiris), and
several cube datasets from NED.  See the following for details and
status; http://wiki.ivoa.net/twiki/bin/view/IVOA/SIA2VAOPrototype