Remove Three Zone Code and Examples

[dhensle]

This pull request removes all code and configuration related to the "THREE_ZONE" system's TVPB (Transit Virtual Path Builder) logsum caching and processing from the ActivitySim codebase. The changes simplify several model steps, eliminate specialized caching logic, and update workflow and settings references accordingly.

Removal of THREE_ZONE TVPB logsum logic:

• Deleted all TVPB logsum caching and computation code from activitysim/abm/models/atwork_subtour_mode_choice.py, activitysim/abm/models/tour_mode_choice.py, and activitysim/abm/models/trip_destination.py, including the creation, updating, and use of TVPB logsum skims and constants. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Workflow and settings updates:

• Removed the initialize_los.py file, which contained all logic for initializing TVPB caches and attribute combinations for THREE_ZONE systems.
• Deleted the initialize_los step from the settings checker in activitysim/abm/models/settings_checker.py, preventing references to its settings file.

Configuration cleanup:

• Removed THREE_ZONE-related regions from the .github/workflows/core_tests.yml file, eliminating test runs for obsolete configurations.

General codebase simplification:

• Removed unnecessary chunk logging and TVPB logsum handling in trip destination logsum computation, further streamlining the code.

Example models removed:

• Marin 3-zone example
• placeholder 3-zone example (based on Marin 3-zone model)
• SANDAG xborder 3-zone example