[trans] - Model comments

[CresitelloDittmar, Mark]

I think I'll start the thread, so the discussion can start to happen before
the model gets to PR...

In another thread, the discussion regarding TOperation being a DataType or
ObjectType became a question.
There, Gerard makes some other statements which, I'm concerned, may pose a
larger concern with the migration of the model through the standards
process:
  * "Actually, it seems to me from
https://volute.g-vo.org/svn/trunk/projects/dm/STC/Trans/doc/WD-Trans-1.0.pdf
that the model still misses a “prescription how axes or so are represented
as variables". *To me the essential elements for a transformation model
would be representation of the actual “things” that are being mapped or
transformed and how these are represented in the operations.* (I have some
initial suggestion how that can be modelled if you’re interested)."

  * "The fact that you think you need it here may be due to an
incompleteness in the model as I described in my previous email.

It may well be that *the structure of the mapping, including explicitly
representation of what-is-mapped-to-what*, could make the need for an
explicit indication of forward vs inverse redundant.
So looking forward to seeing more details of the model (and I can comment
on another thread)."

------
The Transform model describes the relation between 2 CoordinateSystems (
'domain space' + 'frame' ) via Mapping-s.
  * the part being added to Transform is merely an object which links 2
coords:CoordSys objects with a Mapping.
*It does not, and I don't expect to, include descriptions of the 'things
that are being mapped'.*  Whether it is being applied to a Coordinate,
Measure, Region, etc.. is irrelevant to this model.

The Cube model currently has the concept of VirtualMeasure, which says:
   "This Measure is defined by applying Mapping A to a 'source' Measure
type."
This is in Cube rather than Measure, because that is the place where it
became needed.

Mark
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