Support VOSI 1.1 detail=min

[msdemlei]

pyvo currently always parses the full tables endpoint; that can be quite a mouthful, in particular when we are talking about VizieR. But even for GAVO's RegTAP service (http://dc.g-vo.org/tap/tables), that's about 2M of data.

Most of that will be columns of tables any particular user probably will not look at.

To make that a bit more efficient, VOSI 1.1 (https://ivoa.net/documents/VOSI/20170524/REC-VOSI-1.1.html) defines the detail parameter to the tables endpoint. Basically, you get empty table elements when passing detail=min, as in http://dc.g-vo.org/tap/tables?detail=min (which is 146k on the GAVO service at the moment).

As someone accesses metadata of a particular table, you can fill in the columns by requesting tables/table-name.

It would be really nice if dal.vosi.VOSITables could support this. The trouble is that we'd have to put some I/O into the class itself rather than constructing it with the full endpoint content.

Thoughts?

[msdemlei]

While looking for something unrelated, I noticed that pyVO probably already has all on board to use VOSI detail=min; it looks as if p.dal.vosi.VOSITables supports minimal answers as-is, and there are tests for that, too (see dal/test/data/tap/lazy-table?.xml).

Hence, I think all that's necessary to fix this bug would be to pass a detail=min parameter to the requests in TablesMixin. The way it's designed in VOSI, that ought to be safe to do blindly. We could be particularly conservative and check whether there's a version="1.1" (I suspect TOPCAT does this) on the interface of the ivo://ivoa.net/std/VOSI#tables capability, but I'd say that would be overdoing it.