Passenger.reduce() 를 적용하여, 승객을 나열하듯이 넣을수 있게 함.

```python
# 아래는 모두 같은 표현.
# 어른2명, 아이1명
k.reserve( train, [ AdultPassenger(2), ChildPassenger(1) ] )

k.reserve( train, [ AdultPassenger(), AdultPassenger(), ChildPassenger() ] )
k.reserve( train, [ AdultPassenger(2), ChildPassenger(2), ChildPassenger(-1) ] )
k.reserve( train, [ AdultPassenger(2), ChildPassenger(1), SeniorPassenger(0) ] )
```