[task #15637] Match RA and Dec catalog to X and Y catalog to find WCS

[Mohammad Akhlaghi]

Follow-up Comment #5, task #15637 (project gnuastro):

Wonderful review! Thanks Sachin ;-).

About object detection, indeed! That is correct! Gnuastro is founded on the
Unix Philosophy <https://en.wikipedia.org/wiki/Unix_philosophy>, I especially
like the original four principles of Doug McIlroy
<https://en.wikipedia.org/wiki/Unix_philosophy#Origin>. An astrometry program
shouldn't have to worry about how to do detection, that is another program's
job ;-)! 

The input to our program should just be an X-Y-magnitude table and a reference
RA-Dec-magnitude table and its output should be the WCSLIB-created WCS
keywords with distortions, that is all ;-).

The second point is also interesting! I'd love to learn more about it (if only
I had time!). But hopefully later as my schedule opens up a little, I'll dig
into your implementation and learn more ;-).

On the third point, today we have ESA's Gaia survey
<https://en.wikipedia.org/wiki/Gaia_(spacecraft)> which is state of the art
and much more accurate than the ones in that 2009 paper ;-). It has become the
defacto standard for astrometry! That is where I got the reference RA-DEC
catalog that you can use below.

Regarding generating a healpix grid, fortunately there is a standard software
for it: HEALPix <https://healpix.sourceforge.io>, which also has a C library
<https://healpix.sourceforge.io/pdf/csub.pdf>. We can safely assume this as a
new optional dependency of Gnuastro. So if the users don't have it, this
feature won't be built. So go ahead and learn/use it ;-).

I agree, verification is a little more subjective and we can worry about that
when we get an initial fit, its too early to spend time on it now ;-).

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