Move ProfileColorProperty references out of Particle.

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In Particle.ts:

  public readonly colorProperty: ProfileColorProperty;
  public readonly highlightColorProperty: ProfileColorProperty;

Investigate whether these can be moved out of Particle. They are references that are passed in via the constructor, but in light of PhET-iO instrumentation and how that will impact Particle, it would be wise to move them elsewhere if possible.

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This was unnecessary for accomplishing #231 (PhET-iO instrumentation of particles). The colors are provided by the constructors for subclasses HeavyParticle, LightParticle, DiffusionParticle1, and DiffusionParticle2. So the colors do not need to be serialized.