[task #15713] Make quad structure to make unique hashes of 4 objects

[Mohammad Akhlaghi]

Follow-up Comment #24, task #15713 (project gnuastro):

Indeed, its great that the polygons are now distributed over the full field
:-D.

About your first question, please add sorting of the vertices in the
ds9-region function. Otherwise in later scenarios you may mistakenly forget
and will have to spend frustrating hours debugging the problem!

The fact that many of them look like triangles is a problem (because some
images may not have the necessary resolution to separate stars that are so
near to each other. 

You can modify the vertice selection function to be a little more realistic:
keep using the nearest and farthest points (2 points of 4). But for the other
two points, come closer to the center of the distance distribution. For
example, for the third point, try the middle element (at median distance), and
for the fourth point, use the star at the three-quarters of the distance
distribution (quantile of 0.75).  

Another good test would be to change the grid size (right now it is 5 grid
elements per dimension) to smaller grid elements to see if everything is
consistent. For example try 10 or 20 grid elements per dimension and look at
the quad shapes and region file (also post it here). 

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