Allow vehicle to have max speed

[nreinicke]

Our current time traversal model uses a speed lookup table to determine the vehicle's speed on any given road link. But, in some instances of "eco-driving" the vehicle might be better suited in driving a speed that is lower than the speed reported by the road network. For example, on an interstate with free flow speeds at or above 80 mph, a battery electric vehicle might save significant energy by reducing its speed by some amount. To that end, we could create a new time traversal model that would permit the vehicle to drive at some percentage of the road network speed. This should be user configurable and smart such a vehicle is not permitted to drive at speeds that are unrealistic (think 20mph on an 80mph interstate).