[task #16282] Include ICRS equatorial coordinate type

[Mohammad Akhlaghi]

Follow-up Comment #2, task #16282 (project gnuastro):

With the new 'gal_wcs_coordsys_convert_points' that was added in
[https://git.savannah.gnu.org/cgit/gnuastro.git/commit/?id=de51acf8cf1
de51acf8cf1], this task becomes much more easier (to use the same three Euler
angles that WCSLIB uses for converting to-from different coordinate systems).


Also, while looking into this, I noticed that [https://iausofa.org SOFA]
(Standards of Fundamental Astronomy; published by IAU) contains the three
angles from Galactic to ICRS in its g2icrs.c file: 


P = 192.85948    right ascension of the Galactic north pole in ICRS
Q =  27.12825    declination of the Galactic north pole in ICRS
R =  32.93192    Galactic longitude of the ascending node of
                 the Galactic equator on the ICRS equator


For such constants, it may be good to write a small program that uses SOFA
during bootstrapping to build all the constants we need for coordinate
conversion in Gnuastro (just a brainstorm now).


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