Obstacle avoiding turtles

These were Maik and my initial attempts to create sensors and brains and some wall following behaviour. It doesn't work yet, but please let me know which direction (aha!) you want the code to go in and I'll clean it up.

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Added concept of obstacles that are turtles with the ability to check if they contain a specified position.

Added simple Eye sensor that are placed relative to a turtle's body. The new CleverTurtle uses these to try and avoid obstacles, but fails. StupidTurtle.