Ghost zones should only fill boundary

[yt-fido]

Originally reported by: Matt Turk (Bitbucket: MatthewTurk, GitHub: MatthewTurk)

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Instead of cascading interpolation everywhere, the call to retrieve_ghost_zones should only fill the boundary. For codes like Enzo which are not well-behaved, this may mean a much bigger searching process than for codes like FLASH.

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• Bitbucket: https://bitbucket.org/yt_analysis/yt/issue/363

[yt-fido]

Original comment by chummels (Bitbucket: chummels, GitHub: chummels):

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Removing milestone: difficult (automated comment)

[yt-fido]

Original comment by Matt Turk (Bitbucket: MatthewTurk, GitHub: MatthewTurk):

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Oh, I think we might be on the way to this being done for 3.3! The
optimizations do nearly this. Not sure it's worthwhile to close it, but
it's getting near.

[munkm]

@matthewturk I think you were interested in working on this so I've assigned it to you. I know we've had some good work on ghost zones going on (e.g. #2425), but I think this particular issue could use some TLC.