[feature] keep the original order of vehicle journeys

[woshilapin]

Using a HashMap to filter the VehicleJourney then iterating again over them makes the final order of VehicleJourney completely random compare to the input order. This is not really desirable for two reasons:

• why modifying the input order at all? I wouldn't think this would be expected from an end-user
• running twice the same binary might actually produce 2 different orders due to the fact that HashMap is randomized at runtime: it might actually cause some problems for testing

Note that we're talking about transit_model library end-users (using Model). The binaries will also write the output files in different order but semantically, the model is still the same so it's not incorrect.

This modification should barely impact the performance since this is not really changing how the algorithm works.

🔍 Hint - to ease the review, hide whitespace changes