code: def pad_position_w(self, i):
          
          if i >= self.n_pads_w:
              raise ModelError("pad index out-of-bounds")
          return (self.width - self.pad_width) / (self.n_pads_w - 1) * i + self.pad_width / 2
docs: Determines the position of the ith pad in the width direction.
          Assumes equally spaced pads.
  
          :param i: ith number of pad in width direction (0-indexed)
          :return: