[obs-tap] draft summary topic 5: obs_publisher_did

[Mireille Louys]

Topic 5: obs_publisher_did
The document now states it is a unique identifier for a data set , in  
the scope of a publisher 's name space, and in the IVOA , because of  
this identifier uses the concatenation with the publisher_did string  
as prefix.
We still need example to illustrate usage of this field with other  
data model fields such as obs_title and  access_format .

I guess we agreesd that this column is mandatory and searchable ( no  
CLOB) while the url (access_reference) for a data product is not  
searchable.

--ML