Generalize ModelRoadwayNetwork Class

[e-lo]

Generalize the ModelRoadwayNetwork class, moving any agency- or vendor-specific methods to functions or sub-classes.

• [ ] Move vendor- and agency-specific actions to sub-class methods or functions.
• [ ] Create tests for each ModelTransitNetwork method and first-order functions in test_transit.py.
• [ ] Create agency-specific tests for each method in agency-specific sub-classes e.g. test_metcouncil.py
• [ ] Create vendor-specific tests for each method in vendor-specific sub-classes e.g. test_cube.py