updateAlpha(halfLife) {
    goog.asserts.assert(halfLife > 0, 'expected halfLife to be positive');
    this.alpha_ = Math.exp(Math.log(0.5) / halfLife);
  }
Update the alpha with a new halfLife value.

@param {number} halfLife The quantity of prior samples (by weight) used
  when creating a new estimate.  Those prior samples make up half of the
  new estimate.