New UCD1+ for SIA protocol

[Alberto Micol]

After a first glance: I like it! Well, this is not really helping 
anybody out there, is it?

Only two points (in the temporary absence of an established data model):

> VOX:Image_Scale
> --> instr.scale
> with the array value giving the scale in degrees per pixel of each 
> image
> axis.
>

Wouldn't it be better to have it as --> pos.wcs.scale ?
This parameter has to do with the way the data are sampled,
and any software could resample it (eg drizzle) onto a different grid
than the original instrumental pixel scale (and think of the case where
data from different instruments are mosaic'd together).
In the end the instrument (instr.) has nothing to do with the pixel 
scale.

And, anyway the pixel scale can be computed from the pos.wcs.cdmatrix...
why do we need it? Probably for queries ...

Unless you meant the actual resolution (Rayleigh, seeing, etc)
Yet again in this case instr. is not the place to store it.

Actually, where is the resolution in this scheme?
Probably it was not in the SIAP, but I should read the SIAP specs again.


> VOX:Image_AccessReference
> --> meta.ref.url
> specifying the URL to be used to access or retrieve the image.

This is complex ... there could be various URLs associated to
the same image, e.g., to access the full product or only a preview;
but I need to read the SIAP doc again before insisting on this.

Alberto