sheet：当前表格
row_index：单元格行
col_index：单元格列
如果判断是合并单元格，则返回合并单元格的值。
def get_merged_cells_value(sheet, row_index, col_index):
"""
先判断给定的单元格，是否属于合并单元格；
如果是合并单元格，就返回合并单元格的内容
:return:
"""
merged = sheet.merged_cells
for (rlow, rhigh, clow, chigh) in merged:
if (row_index >= rlow and row_index < rhigh):
if (col_index >= clow and col_index < chigh):
cell_value = sheet.cell_value(rlow, clow)
# print('该单元格[%d,%d]属于合并单元格，值为[%s]' % (row_index, col_index, cell_value))
return cell_value
break
return ''