[SpectralDM] - UTypes changes

[CresitelloDittmar, Mark]

Doug,

The utype list of the latest draft should be pretty similar to the previous
version.
The Change Log is fairly detailed about what was modified.

All changes were in response to the request for these models to share a
core data model and are founded on the DM list discussion from Nov->Jan
on the separation of Observation-Dataset (see ImageDM Twiki).

The Cube model work has gone a step further, to separate Dataset from
ObsDataset,
but is completely compatible with what is in Spectral.  My expectation is
that
with the completion of the Cube model work, a definitive
"Dataset/ObsDataset Metadata" model
would be established (either in that doc, or as a separate one) for use in
Cube/Spectral/ObsCore etc.  The Spectral doc would be updated to remove that
portion from its content, and reference this other.

Mark
=====================================================

In Response to:
The same point should be made about the SpectralDM 2.0.  I just checked,
and the Utypes in the latest draft do not agree with ObsCore or Char.

There was a time once, about 3 years ago, when we were trying to fast
track an update of the SpectrumDM to a generalized SpectralDM and wanted
compatibility with the existing standards and implementations at the
time.  However if are still now finalizing SpectralDM 2.0, and have
decided that ImageDM should extend ObsCore, I suggest that the new
SpectralDM should do so as well.

All of these primary data models - ObsCore/Char/ObsTAP, ImageDM, and
SpectralDM, should at this point share the same core data model based
upon the latest Char2 Utypes.  We already pretty much reached that point
last fall.
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