code: private double[] getAugmentedResponse(double[] y) {
          double[] ret = new double[y.length + p];
          System.arraycopy(y, 0, ret, 0, y.length);
          return ret;
      }
docs: transform the original response array by padding 0 at the tail
  
  @param y
  original response array
  @return response array with padding 0 at tail