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* Early termination for find_{min,max}_global

This patch adds a callback to allow the user to request cancellation of a
search using find_{min,max}_global.  This enables users to cancel
searches when they are no-longer relevent, or when the user has some
special knowledge of the solution that they can use the stop the search
early.

closes  #2250

* Moved default stopping condition into find_max_global.h since that's the code it relates to and did some minor cleanup.
Co-authored-by: Davis King <davis@dlib.net>