This pull request removes reason and code used only by reason from yt.
The yt.gui submodule
The vm_panner (yt/visualization/image_panner)
Reason has been disabled since the yt 3.0 release and has not been updated. If anyone wants to update it in the future, the code lives on in the version control history.
If anyone objects to any of the code I'm removing or changing, please let me know. If you feel that I should bring this up on yt-dev, I'm happy to send out an e-mail about it.
I think anytime a PR is issued that is removing functionality (even if its broken functionality) from the codebase, there should be a notification on the yt-dev (and potentially yt-users) list, since not everyone reads all of the PR announcements regularly.