Convert vehicle type choices to integers in ABM3

[bhargavasana]

This is to remove string choices in the vehicle type choice model based on work already done for MWCOG. There could potentially be some memory efficiency gained. Appropriate config files (maybe even for some downstream models) need to be updated also. Here are some examples of config files that need to be udpated:

• https://github.com/SANDAG/ABM/blob/ABM3_develop/src/asim/configs/resident/vehicle_type_choice_op4.csv
• https://github.com/SANDAG/ABM/blob/ABM3_develop/src/asim/configs/resident/vehicle_allocation.csv

[bhargavasana]

MWCOG work will come in with Sharrow enhancements later. #96 is good enough to close this for now.