Change references to Location to UnLocode in domain objects

[paulrayner]

Location is an aggregate root, and there are references to it all over the place, including from Leg, which is a value object. Would like to try changing all these to use the UnLocode value object, which is the business key for Location.

This change is a deviation from the original DDD sample app and will likely mean more calls to LocationRepsitory (which could be cached if needed, since there are not that many values), but should make working with all the aggregates a lot cleaner by making the aggregate boundaries more well-defined.