ENH: The parameter merge_cells parameter in function to_excel should take a 2-tuple one for index and one for columns
Currently pandas `to_excel` function has one parameter named `merge_cells`: if `True` it merges all cells in the excel sheet corresponding to the index and columns. However, if there are many levels of the index and many values for the higher levels, then as we scroll on the sheet, values of the higher levels in index become invisible. One way to keep the repeated values is to set `merge_cells = False`. However, in that case the column levels are also affected.

It would be great to have a way to control this aspect of index and columns separately. One way would be to let `merge_cells` to take a 2-tuple of which one value is for the index and the other for the columns.