Markus created this in Sept 2018 on realising how difficult it was to
furnish some catalogue that gave HD numbers with modern astrometry --
VizieR's HD catalog is really hard to find, and the Japanese service
that's easy to find sucks.  Neither had a link to Gaia.

So, I did what's described in q.rd's _longdoc

The test set is generated by selecting HD 199535 and whatever else of
its surroundings (that's because of our Gaia test set).  You'll need
the full data/dr2-match.vot and data/dr3-match.vot during import.