Visualization of GTU generators and generator functionality

[WJSchakel]

Visualization of GTU generators currently does not work. In the past we had number of GTU's in the queue being displayed. But as we have no formal definition of what a GTU generator should be able to report, a change in the code resulted in a TODO pending a better definition. Note that essentially any object can place GTUs on the network, and that such objects may not represent something with a specific location. For example, a generator could randomly put GTUs on the network in some neighborhood area. Others can split the demand over a set of connectors, and generate GTUs at the start of a few connected links. And yet others simply operate at the start of a single lane.

[WJSchakel]

• Changed GtuGenerator to be able to report queue lengths at one or more Locatables. The GtuGenerator itself is a NonLocatedObject.
• Added GtuGeneratorAnimation, with a nested Queue class. The generator is animated as 3 blue chevrons, with an optional queue number (similar to how IDs are shown for objects).
• Removed GtuGeneratorQueue and GtuGeneratorQueueAnimation. These didn't have a logical located/non-located structure.
• Removed ADD_GENERATOR_EVENT and REMOVE_GENERATOR_EVENT from the network. These were not used. Generators can be added as either LocatedObject or NonLocatedObject. Depending on the type, different animation may be created.

The Detector class has been renamed to LaneDetector, and a Detector interface is introduced. It extends LocatedObject. In this way a TrafficLightDetector (which is not a LaneDetector but uses 2, specifically StartEndDetector) can also be animated. The default animation factory purposefully ignores the StartEndDetector type, as the TrafficLightDetector is animated as a whole.