[task #15803] Match program should build k-d tree and later read from it

[Sachin Kumar Singh]

Follow-up Comment #1, task #15803 (project gnuastro):

I'm about to begin with the implementation of this task and just wanted to
discuss the details of the implementation. 

So, we simply have to add a --kdtree option in the match program which takes 2
parameters. If the parameter is `build`, we will make a KD tree using the X, Y
coordinates in `large.fits` using the `gal_kdtree_create` function and we can
output the kdtree in a file using the --output option.

Next for matching the coordinates in the `small.fits` it will use the
`gal_kdtree_nearest_neighbour` to match each point in the `small.fits` to the
nearest neighbor in the kd-tree. 

If the above case is okay, I think the command should look as below, because
now the call to `large.fits` is redundant as it is used to create kdtree which
has already been done.

astmatch small.fits --ccol1=X,Y --kdtree=kdtree.fits


But I also understand that using the command below is more similar to how
`astmatch` it is normally used and hence much more familiar to the users.

astmatch large.fits small.fits --ccol1=X,Y --ccol2=X,Y \
         --kdtree=kdtree.fits


What do you think?

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