The agents auto-update themselves and can calculate their velocities.
This keeps the user from having to create an update_agent function.
It can also save the user from having to keep track of and update
velocities at all by using the provided `apply_steering` method.
Closes#15, closes#16
The use of `not is_inside_tree()` before setting the setting class' value
meant that the starting values would always be the default. Moving the
value setting before checking for tree readiness fixes the issue.
