remove variable observation_space since it is not really used anywhere and adjust examples in doc to match the code

Refactoring: add return types for ObservationBuilder.get(self, handle: int = 0) and ObservationBuilder.get_many(self, handles: Optional[List[int]] = None)

Refactoring: delete new_position from TreeObsForRailEnv and use flatland.core.grid.grid4utils.get_new_position instead

updated malfunction behavior. Now a train can break at any point during its travel. We don't count number of stops anymore. This is to simulate more complex stochastic events.