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

[Mohammad Akhlaghi]

Follow-up Comment #2, task #15803 (project gnuastro):

Thanks for the nice review, its great to add this feature soon :-).

I think the main thing you missed is this: the k-d tree file should just
include the k-d tree (the raw output
<https://www.gnu.org/software/gnuastro/manual/html_node/K_002dd-tree.html> of
'gal_kdtree_create', which is only two columns). 

So when you want to search with that k-d tree, you need the actual table that
the k-d tree was created with also. 

The reason for this is that k-d trees usually become useful for large tables
and large tables will take space to store (for example assume that
'large.fits' is a 100Mb table). There is no need to force the user to have an
extra copy of the large table inside the 'kdtree.fits'. 

Does this sound reasonable?

About the acceptable strings given to the '--kdtree' option, it just occurred
to me that we may want a third alternative also:

'--kdtree=build': will build the k-d tree, and write it inside the file given
to the '--output' option.

'--kdtree=internal': we will construct the k-d tree internally, use it in
parallel, and free the k-d tree after the match. 

'--kdtree=FILENAME': it will read the k-d tree from the given file, use it in
parallel, and free it. Ofcourse, in this scenario, we need to check that the
k-d tree in the given file has the same number of rows as 'large.fits'. 

By the way, when matching in k-d tree mode, we should always assume that the
k-d tree will be built/used over the first input catalog. 

How does this sound?

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