Inheritance: RectangularWeights
 public ConvolutionStep(IList<RectangularStep> upstream, int convolutionWidth, int convolutionHeight)
     : base(WidthOf(upstream) - convolutionWidth + 1,
          HeightOf(upstream) - convolutionHeight + 1, upstream)
 {
     this.weights = new ConvolutionWeights(convolutionWidth, convolutionHeight, upstream.Count);
 }