Pystk2-gymnasium

- *Multiprocess*: no more limitation on the number of races
- Improved documentation

- Changed default for steer discretization steps (7)
- (breaking) Use multi-discrete for item types

- `center_path_distance` is now relative (to indicate left/right of the path)
- `center_path` gives the vector to the center of the track
- Changed the reward to gives some importance to the final ranking
- Better regression tests

- Fix in formula for `center_path_distance`

- Multi-agent environment
- Use polar representation instead of coordinates (except for the "full" environment)
- Only two base environments (multi/mono-agent) and wrappers for the rest: this allows races to be organized with different set of wrappers (depending on the agent)
- Added `center_path_distance`
- Allow to change player name and camera mode
- breaking: Agent spec is used for mono-kart environments