applyBehaviour(particle, time, index) {
    this.calculate(particle, time, index);

    particle.alpha = particle.data.alphaB + (particle.data.alphaA - particle.data.alphaB) * this.energy;

    if (particle.alpha < 0.001) particle.alpha = 0;
  }
Applies the alpha behaviour to the particle.
@param {Particle} particle - The particle to apply the behaviour to.
@param {number} time - The current simulation time.
@param {number} index - The index of the particle.