fix observable

Target.py

@@ -1,9 +1,12 @@
 from observable import Observable
+from collections import defaultdict
+
 import logging
 
 class Target(Observable):
     def __init__(self, points, name):
         logging.info("Target " + name + " created")
+        self._events = defaultdict(list)
         self.points = points
         self.name = name
         self.hit_key = "hit"