refactor my-od-index

[colinsheppard]

instead of using my-od-index throughout the code, do the following:

• add a variable to driver breed, call if "current-od-index"
• rename "my-od-index" procedure to "update-od-index" which will be a procedure called from a driver context, this procedure will update current-od-index variable using the same formula
• add update-od-index to the code at any point in which the destination taz changes for a driver (so in update-itinerary, but maybe in other places as well)
• now change all of the places where "my-od-index" was used and have them use "current-od-index" instead

Verify that model runs with the changes are identical to without.

[colinsheppard]

For verification, you should be able to run the model using the "setup-and-fix-seed" button which uses a seed of 1 with logging on for a few important log files (log-trip, log-charging, and log-pain should suffice) and get identical results before and after the code changes (here's a good place to do work in a branch first, then it's easy to pop back and forth to compare output).

[arharris]

Changes have been applied; the output logs were identical to the original model.