Argument `sheet_name` is ignored when reading xlsx file with one sheet

[danielhuppmann]

While reviewing #876 by @korsbakken, I noticed that the sheet-name argument is ignored when reading an xlsx file that has only one sheet. This should be fixed such that sheet_name defaults to None and the parsing uses data* only if sheet-name is None.

Basically, when passing sheet_name="foo" and there is no sheet with that name, pyam should raise an error.

Could be implemented jointly with #690