add vehicle_type_id to trips.txt

[BTollison]

This is a proposal to add vehicle_type_id (from issue #30) to trips.txt to be able to identify the type of vehicle meant to operate the trip.

vehicle_type_id
Used to identify a specific vehicle type within a fleet.

• comes from proposed vehicles.txt
• used in proposed energy_consumption.txt in issue add Energy_consumption.txt #29