Interception code taking into account the current speed.

There is a bug in that the robot stops firing after the first enemy is dead.
Change the color of explosion if a hit has been made.
Rudimentary veering left and right. Acceleration could be normalized for us, or we could normalize.

When our velocity is below a certain amount we try and accelerate in the same vector as we are already going.

TODO decide whether to steer left or right. It would be good to do do this based on clockwise/anticlockwise motion.