feat: new "event based collisions" example

[rico-haas]

Summary

This is a playful example that makes use of Mesa's DEVSimulator event-based simulation capabilities on a continuous timeline. It features moving discs in a 2D environment that collide with the border and each other, using an event-based collision mechanism. With help of the simulator, discs will collide accurately, no matter how fast they are!

Motive

This provides an example usage of Mesa's DEVSimulator event-based simulation capabilities on a continuous timeline, see #124. This model has been explicitly designed to demonstrate that simulator feature, since without it, the model couldn't have the same accuracy guarantees (discs could pass through each other).

Implementation

1. Discs are placed in the space with a random movement direction.
2. At the beginning and whenever a disc changes its trajectory, the disc calculates when it could collide with an obstacle (other discs or the border) and schedules these as future events.
3. When such an event occurs and the disc still has the same trajectory (it hasn't collided with something else) then it changes its trajectory based on its angle of incidence.
4. Each full time step, all discs update their shown position based on their current trajectory. This doesn't update the actual model at all, it only queries the current state. Hence the simulation results are always the same, regardless of the timestep interval.

Usage Examples

1. Set the disc speed to 1 and let the simulation run for let's say 100 time steps.
2. Take a screenshot of where all the discs are after 100 steps.
3. Turn the disc speed up to 10 and reset the simulation.
4. Now let the simulation run only 10 time steps.
5. Compare the result with your screenshot. It should be exactly the same!

Additional Notes

This example is based on my idea for a DEVS tutorial.

[Logende]

The tests are failing, but the failures seem unrelated to this PR. @EwoutH how to proceed?

[EwoutH]

This seems really cool! I toyed around in the days I was still using NetLogo with almost exactly a model like this.

I'm a bit busy, but I will try to review somewhere this weekend. Maybe @tpike3 or @quaquel might also find this interesting.