Simulation of a 2D gas of colliding circular balls

Simple simulation of a 2D gas of elastically colliding circular balls of various masses and radii

run_sim2d(N,mass,radius)

Simple simulation of a 2D gas of colliding circular balls.
N,mass & radius are vectors of the same length specifying the
number,masses & radii of the balls.

example : run_sim2d([400 100],[1 2],[0.005 0.01])
creates a simulation with 400 balls of mass 1 and radius 0.005, and 100
balls of mass 2 and radius 0.01

The box coordinates are defined such that min(width,height) is 1 unit.
The simulation is initialized with uniformly random positions and a
constant speed in the horizontal direction.