Chapter 4 Edits

[shiffman]

Just a start so far. . .

[shiffman]

Noting a couple open questions as I pause at section 4.5.

1. Once I add origin this code becomes a little awkward, might reconsider.

  addParticle() {
    this.particles.push(new Particle(this.origin.x, this.origin.y));
  }

2. applyForce() gravity should maybe end up in the ParticleSystem class, but maybe I just address this later in the chapter.

Code updates: nature-of-code/noc-examples-p5.js-archived#93

[shiffman]

@joeyklee also if you look at my changes so far I decided to go with splice() over filter() but keeping a reference to filter(). I should also probably explain that there is a for in loop and a forEach() higher-order function somewhere.

[shiffman]

Just under 7 months later I have returned to editing this chapter! Merging in the final changes and noting small things remaining in separate issues.

[joeyklee]

Hooray! Excellent!