Assure reset of _last_timestep

trajectron/environment/node.py

@@ -31,6 +31,15 @@ class Node(object):
 
         self.forward_in_time_on_next_override = False
 
+    @property
+    def first_timestep(self):
+        return self._first_timestep
+
+    @first_timestep.setter
+    def first_timestep(self, value):
+        self._first_timestep = value
+        self._last_timestep = None # reset
+
     def __eq__(self, other):
         return ((isinstance(other, self.__class__)
                  or isinstance(self, other.__class__))