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

fixed bugs in code but got new assertion error in tests. these need to be investigated before removing

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.