Example particle system

This fixes examples/Topics/Simulate/MultipleParticleSystems. I tried to honor @shiffman's intent regarding polymorphism, and I hope refactoring the boolean logic is a good thing, rather than confusing a learner.

Sorry I didn't squash these five commits into one -- I had already pushed the local branch to github. Let me know if you'd prefer one squashed commit, and I'll create another branch/pr.