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