Game Development Reference
* Keeps track of one force generator and the particle it
* applies to.
* Holds the list of registrations.
typedef std::vector<ParticleForceRegistration> Registry;
* Registers the given force generator to apply to the
* given particle.
void add(Particle* particle, ParticleForceGenerator *fg);
* Removes the given registered pair from the registry.
* If the pair is not registered, this method will have
* no effect.
void remove(Particle* particle, ParticleForceGenerator *fg);
* Clears all registrations from the registry. This will
* not delete the particles or the force generators
* themselves, just the records of their connection.
* Calls all the force generators to update the forces of
* their corresponding particles.