bring the scan update outside the state update

[luigiberducci]

As in the title, bring the update of the scan outside this method, so that update_poseis only responsible for updating the racecar state.

In the future, we could compute the scan only if required by the observation space, so it would be better to separate it.

Note: currently the collision check relies on the scan, so this issue depends on finding an alternative way to check for collisions.

Originally posted by @luigiberducci in #74 (comment)