Define a way to discover s_region usability

Probably the easiest way to fix this would be to say "if you give s_ra and s_dec, you MUST give s_region". That would be my preferred solution, actually.

Alternatively, there should be a way for data providers to say "you will not get extra false negatives if you use s_region over s_ra and s_dec on this table". Ideally, this would not require Registry interaction; and actually, the Registry does not have an out-of-the-box mechanism for something like this either. We would have to define an Obscore registry extension for such a flag, and that seems overkill to me at this point.

I wonder if we should'nt just have a hack and say "if your s_region column is fine, add the literal string 'safe-for-queries' to s_region's description". Yes, this may seem to radiate a bit of bad karma, but it works nicely with both TAP_SCHEMA and VOSI, which is nice and seems hard to attain with just about anything else.

Another alternative that has this property would be to change the s_region's UCD or utype for query-safe (or non-query-safe?) s_region columns... hm.