Some changes for CommonRoad coupling

[WJSchakel]

At https://github.com/CommonRoad/opentrafficsim?tab=readme-ov-file#differences-of-this-fork-to-ots some changes are made to a forked repository for purposes of coupling OTS and CommonRoad. These changes can be incorporated by:

• Creating a version of GtuCreator where the hard-coded values can be set.
• Using FilterDataGtuType and two new classes FilterDataLength and FilterDataWidth.
• Incorporating the HeadwayGenerator class and use it in ShortMerge, StraightModel and DemandParser, which all have highly similar private classes.

[WJSchakel]

org.opentrafficsim.road.gtu.generator.GtuSpawner was added in order to add GTUs from external control.

FilterDataLength and FilterDataWidth added under the road project. A mixture of base and road extended data types and filter data types can now be used in the sampler.

HeadwayGenerator is added and used in the three demos.