Generates the sequence of merged cell ranges in the format:

    ((col_low, row_low), (col_hi, row_hi))

def merged_cell_ranges(self):
    """Generates the sequence of merged cell ranges in the format:

    ((col_low, row_low), (col_hi, row_hi))
    """
    for row_number, row in enumerate(self.raw_sheet.rows()):
      for col_number, cell in enumerate(row):
        rspan, cspan = cell.span
        if (rspan, cspan) != (1, 1):
          yield ((col_number, row_number), (col_number + cspan, row_number + rspan))