seedSystem(numParticles) {
    numParticles || (numParticles = this._totalParticles);
    const particlesEmitted = 0;
    while ((this.particleCount < this._totalParticles) && (particlesEmitted < numParticles)) {
      this.addParticle();
    }
  }
Seeds a particle system with a number of particles immediately, or the max if no number provided.
NOTE: this does not simulate system, only adds particles at initial position
@param {Number} [numParticles=max] number of particles to seed system with, up to max