Merge pull request #623 from dstl/deleter_fixes

Fix time steps based deleter 

Fixes #610

Author: sdhiscocks

stonesoup/deleter/time.py

@@ -27,11 +27,20 @@ def check_for_deletion(self, track, **kwargs):
         Returns
         -------
         bool
-            `True` if track has an :class:`~.Update` with measurements within
-            time steps; `False` otherwise.
+            `False` if track has an :class:`~.Update` with measurements within
+            time steps; `True` otherwise.
         """
-        return not any(isinstance(state, Update) and state.hypothesis
-                       for state in track[-self.time_steps_since_update:])
+        timestamps_set = set()
+
+        for state in track[::-1]:
+            timestamps_set.add(state.timestamp)
+            if len(timestamps_set) > self.time_steps_since_update:
+                return True
+
+            if isinstance(state, Update) and state.hypothesis:
+                return False
+
+        return False
 
 
 class UpdateTimeDeleter(Deleter):
@@ -57,8 +66,8 @@ def check_for_deletion(self, track, timestamp=None, **kwargs):
         Returns
         -------
         bool
-            `True` if track has an :class:`~.Update` with measurements within
-            time; `False` otherwise.
+            `False` if track has an :class:`~.Update` with measurements within
+            time; `True` otherwise.
         """
         if timestamp is None:
             timestamp = track.timestamp