//If the airport already has a passenger with that username,
//throw an InvalidOperationException with the message "Passenger {username} already registered!".
//The command adds a new passenger into the airport and returns "Registered {passenger.Username}"
public string RegisterPassenger(string userName)
{
IPassenger passenger = airport.GetPassenger(userName);
if (passenger != null)
{
throw new InvalidOperationException($"Passenger {userName} already registered");
}
passenger = new Passenger(userName);
airport.AddPassenger(passenger);
return($"Registered {passenger.Username}");
}