Define a listener. This is a user-friendly wrapper for
the class constructor Listener$new(). Users who are not familiar with
the concept of classes may consider using this wrapper directly.
Listener is an important concept of TrialSimulator. Used with a
trial object in a controller, a listener can monitor a running trial to
execute user-defined actions when it determine condition of triggering a
milestone is met. This mechanism allows the package users to focus on
the development of action functions in a simulation.