Make coverage optional

[jd-au]

SSA 1.1 sec 4.2.5.10 states MANdatory coverage information (Char.SpatialAxis.Coverage.Location.Value + 2 others),
but they are missing from example in Appendix A
. Appendix A is output from an SVO service; that service now fixed to contain (blank) missing columns

Theoretical services cannot provide coverage information so these columns should be made optional to support theoretical services.

Reported by @mbtaylor in the OPS Progress discussion https://wiki.ivoa.net/internal/IVOA/InterOpOct2022Ops/ops-progress.pdf in the https://wiki.ivoa.net/twiki/bin/view/IVOA/InterOpOct2022Ops session.

[jd-au]

I've updated §4.2.5.10 to mark the previously mandatory fields as recommended and added the following text:

This component is optional to support theoretical services which do not have coverage information. However, where coverage information is available, such as in observational services, the columns Char.SpatialAxis.Coverage.Location.Value, Char.SpatialAxis.Coverage.Bounds.Extent, Char.TimeAxis.Coverage.Location.Value, Char.SpectralAxis.Coverage.Location.Value, and Char.SpectralAxis.Coverage.Bounds.Extent should be provided.

[jd-au]

I've created a draft erratum at https://wiki.ivoa.net/twiki/bin/view/IVOA/SSA-1_1-Err-4

[mbtaylor]

The Erratum looks good to me.

[acraugh]

As noted in the TCG meeting (19 May 2024), I would suggest inverting the paragraph so that the 90% case where coverage should be supplied is addressed first, and the exceptions where it is reasonably omitted come second. The change itself seems like a reasonable.