code: def add_background(image, sigma_bkd):
      
      if sigma_bkd < 0:
          raise ValueError("Sigma background is smaller than zero! Please use positive values.")
      nx, ny = np.shape(image)
      background = np.random.randn(nx, ny) * sigma_bkd
      return background
docs: adds background noise to image
      :param image: pixel values of image
      :param sigma_bkd: background noise (sigma)
      :return: a realisation of Gaussian noise of the same size as image