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 rlo, rhi, clo, chi in self.raw_sheet.merged_cells:
      yield ((clo, rlo), (chi, rhi))