made diameter a public property of TWLBall so the sockets can computer GAP between their head and the ball
added radius property to all sockets which is defined at the same time as the physics body on the head node
TWLEaterSocket now computes the GAP between its head and the ball by calculating the distance between their centres (which is the position of each of the nodes in the scene coords) and then subtracting the ball and head radii.