map loading and track-object

[luigiberducci]

Hi,
I wanted to discuss the map loading and its extension with you.

The current implementation requires specifying many arguments (e.g., map_path, map extension, ...) and
offers a limited collection of maps in the dir envs/maps.
Moreover, I think would be convenient to think of a way to access the centerline/raceline, instead of loading them from a file.

I would propose:

1. adopting the structure of f1tenth/f1tenth_racetracks as default, for example adding it as a git submodule.
2. to create a track object, from which the user can easily access centerline/raceline and other map-related utilities.

What do you think?