Reintroduce POLYGON for proper motions, requires new operator POLYGON_PROPER_MOTIONS

[HannoSpreeuw]

The original Transform and Select chapter contained a beautiful application of the convex hull algorithm from Scipy.
This was used to derive a polygon which could be included in an ADQL query to select stars based on their proper motions.
However, POLYGON from ADQL is not meant to select based on proper motions, it should only be applied to positions on the sky.

Therefore it has recently been replaced by two BETWEEN operators

POLYGON for proper motions will still work in our case, with this trick

AND 1 = CONTAINS(POINT(-pmra, -pmdec),
                   POLYGON({negative_pm_point_list})) 

But that will not work in all cases, as @abostroem pointed out, so is not recommended.
At this point the two BETWEEN operators are the best we can do, it seems.

But I would like to reintroduce the convex hull and more elegant selection of stars based on proper motions as soon as something like a POLYGON_PROPER_MOTIONS operator has been built.

Hopefully VO people will build such an operator in the near future.

@eteq, @abostroem mentioned your name in the Slack channel, could you please keep us updated on any developments with respect to this matter?