bugfix #141: check_path_exists and tests

examples/custom_railmap_example.py

@@ -43,6 +43,7 @@ def custom_schedule_generator() -> ScheduleGenerator:
 env = RailEnv(width=6,
               height=4,
               rail_generator=custom_rail_generator(),
+              schedule_generator=custom_schedule_generator(),
               number_of_agents=1)
 
 env.reset()