-PuPy.RobotCollector has optional argument vars with a list of variable names that are written to the log file (loosely tested).
-PuPy.RobotActor has child as optional argument (makes sure child is always callable)
-PuPy.RobotActor has member method _get_initial_targets that returns a target iterator as in __call__
-PuPy.WebotsRobotMixin uses actor's _get_initial_targets method instead of calling the actor before the run loop.
-refactored all supervisor checks. They are now composed of two subclasses, one for the check, one for the action to be executed (loosely tested).
-minor changes