update_highway_project_years - reference "completion year" instead of "scenario code"

The schema of the "bus_future" feature class has been changed to include a completion year, and similar changes are occurring in the MRN. 

Fix "update_highway_project_years.py" to look at completion year instead. All references to scenario codes in the MHN should occur through MHN.scenario_years within MHN.py.