Implement parsing xml file with Intersection data
Rename TrafficSignal class to TrafficSignalState

This change is made because the name TrafficSignal is confusing since there is
also a class named TrafficControl. Thus, TrafficSignal class is renamed to
TrafficSignalState because it represents the state of the traffic signal.
Add setters/getters for Car and GridNode class
Rename ambiguous variable name

Variable "index" represents the location of the traffic signal. However, it is
hard to tell what the variable is used for just by looking at it's name. Thus,
it is renamed to something more relevant.
Remove unused import
Rename TrafficType class to IntersectionType

TrafficType implies the type of the cars instead of the type of the
intersection.
Use comprehensible method name and variable