ENH: Excel - allow for multiple rows to be treated as hierarchical columns
related #4468

Add keyword argument to ExcelFile parser to take an integer / list for the rows to interpret as header rows. If more than one, interpret as hierarchical columns / MultiIndex.

Presumably this would also allow you to round-trip  Data Frame with hierarchical columns.

Basically, if you have something spanning two columns, just converts to two cells with the data in the original cell, ending up just like what you need for csv reader.
