Throw an exception when a up/down train is inserted as a down/up (opposite direction) train
Create bobura::model::train::is_origin_stop(), is_destination_stop(), previous_stop() and next_stop()
Implement bobura::model::train::is_origin_stop()
Implement bobura::model::train::is_destination_stop()
Change the specification of bobura::model::train::is_origin/destination_stop()
Change the specification of bobura::model::train::previous/next_stop()
Change bobura::model::train::is_origin/destination_stop() to origin/destination_stop()