accumulateAdditive method
Implementation
accumulateAdditive(int weight) {
  var buffer = this.buffer,
      stride = valueSize,
      offset = stride * _addIndex;
  if (cumulativeWeightAdditive == 0) {
    // add = identity
    _setIdentity();
  }
  // add := add + incoming * weight
  _mixBufferRegionAdditive(buffer, offset, 0, weight, stride);
  cumulativeWeightAdditive += weight;
}