Custom map without OpenDrive

[drbswa]

I think it would be nice to add the ability to add custom maps without requiring OpenMap data. It would open up the use cases to things like off-road driving, which has had an increase in interest lately.

[sguttikon]

As workaround, i create an empty openmap data. But I also had to get rid of some functionalities such as traffic manager.
#3623 (comment)

<custom_map>/OpenDrive/<custom_map>.xodr content:

<?xml version="1.0" encoding="utf-8"?>
<OpenDRIVE/>