Merge pull request
#4412 from jdfreder/traitlet_notify_fix...
Merge pull request
#4412 from jdfreder/traitlet_notify_fix
Fix traitlet _notify_trait by-ref issue
This fixes a problem with the existing traitlet machinery, where anytrait values get added to the notifiers list every time the _notify_trait function is called.
The bug is that each time the traitlet changes, the anytrait callbacks will each be called N times, where N is the number of times the traitlet has been changed (in the current application instance).
