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

[Sachin Kumar Singh]

Follow-up Comment #10, task #15713 (project gnuastro):

The quads were made by firstly doing a search for all points and calculating
the distance from every other point and filtering out those which were less
than the helpix resolution(theta-pix as given in the documentation). It took
an O(n^2) time and O(n) space complexity. 
After that, I sorted(O(k*log(k)) these points(for every star) on the basis of
distance from itself(including itself which produces distance = 0). 
After that, if the array has at least 6 elements, then I pick the n-1, n-3,
n-5 and 0 indexed elements and sort(O(4*log(4)) them according to brightness.

Finally, I make an array for plotting them and making a region file.
For viewing the output many printfs are included in between for debugging. We
can do a meet where I can briefly explain all the code:-)
For the counting of the number of times a star can be used, I included a
variable called `times_used` in `stuct quad_candidate`. 

Fo generating helpix, I've created a function `make_healpix_polygon`. It
generated a region file called polygon.reg for healpixes. 

The polygon.reg has helpixed and final-quad.reg has final quads used. For now,
some of the polygons in final-quad.reg might not be a proper polygon as they
are still sorted on basis of brightness. To make them perfect polygon we can
use the polygon sort function from the polygon library. 

(file #49469, file #49470)
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Additional Item Attachment:

File name: polygon.reg                    Size:1 KB
    <https://file.savannah.gnu.org/file/polygon.reg?file_id=49469>

File name: final-quads.reg                Size:1 KB
    <https://file.savannah.gnu.org/file/final-quads.reg?file_id=49470>



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