Obscore 1.1 Erratum 3: Drop obs_id non-NULL requirement

[Markus Demleitner]

Dear DAL, dear DM,

[followups again suggested to DAL only]

Given that the discussion on the non-NULL requirement on obscore's
obs_id column did not show a convincing need for the requirement and
it is an implementation burden in particular in the presence of
validators (see the erratum's rationale), I've written up a proposed
erratum for Obscore 1.1; see

https://wiki.ivoa.net/twiki/bin/view/IVOA/ObsCore-1_1-Erratum-3

Does anyone have major qualms with this?  Have I perhaps missed 
material in the Obscore document that would also need changes?

      -- Markus