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
Used `var variable := 0.0` as discussed in the Godot issue, instead of
`var variable: = 0.0`.
Mostly these are minor/cosmetic changes, but I've also reorganized the
folder structure (naming of folders) to reflect our guidelines, plus
made some minor changes to the demo codes. Still work in progress.
