Particle System "origin" and addParticle()

[shiffman]

nature-of-code/noc-book-2#75 is now merged but one thing I want to go back and look at is how the origin is stored and intialized with a particle system. Overall I find this passing in x,y and then creating a vector more clear than passing in a vector and calling copy(). In addition, some of the example there isn't a good reason to store an origin and it would make more sense to call addParticle(x,y).