This PR resolves #112 by adding partial support for Path Vehicles.
By "partial support" I mean everything in the official geoscenario spec is implemented except for:

• agentacceleration and timetoacceleration tags for path nodes.
• cycles and usespeedprofile tags for vehicles. I don't think the existing TV implementation supported these vehicle tags either.

This PR contains the following changes:

• Add a new PV class of type Vehicle
• Update the Actor class to include a new follow_path function similar to the existing follow_trajectory function
• The new follow_path function uses the frenet distance s to travel along the defined path, which also allows the PV to provide some data in frenet frame as well
• Updates the dashboard to be able to display frenet frame information without the rest of the debug data
• Adds a brief scenario for testing PVs
• Cleans up some broken btree references and deletes some old duplicated btrees