Obstap / about optional fields

[François Bonnarel]

It was admitted a long time ago that the obstap parameters should alllow 
the same ADQL
query to be performed on all Obstap/compliant services. Therefore a 
limited set of mandatory
parameters has been defined covering most of the Initially proposed use 
cases.

However a few of them (polarization data, Cubes with Radial Velocity, 
specific dataproduct subtypes, etc...) were missing.
A small set of additional optional parameters have been defined in the 
document. (Some new ones have been proposed
in the current discussion)

I see two interests in these optional fields:
     * complete the description and help the data discovery phase by 
displaying more complete descriptions of the data sets

     * extend the Obstap Query mechanism for the benefit of small 
specialized communities...
ADQL query on optional field will make no harm to "minimal" Obstap 
services and will return no
data from them. But data providers of these kind of data will probably 
fill these optional
fields for a better exposition of them, and data consumers may use them 
in their queries if they are interested  and get a chance to discover 
some .....

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