New 4 coordinate crossmatch

[luthienliu]

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[adrn]

Hi @luthienliu! Thanks for this PR. It looks like there are some changes to a FITS-tables tutorial in the changes here. Could you please remove those changes from this PR, since I think they are in #564? You'll want to do an interactive rebase to get rid of those commits -- if you aren't sure how to do that, feel free to message me on the Astropy Slack and I can help you through it. Alternatively, I could do the rebase and push to your branch directly -- just let me know what you prefer.

[adrn]

Hi again @luthienliu - I went ahead and fixed the merge conflicts for you by rebasing your branch, but in doing that I noticed that you made a new duplicate version of the tutorial instead of modifying the existing 4-Coordinates-Crossmatch.ipynb file. Please copy the proposed changes into that file instead.