read_excel and ExcelFile engine conflict
This is currently valid code:

```python
>>> xl = pd.ExcelFile('foo.xlsx', engine='xlrd')
>>> pd.read_excel(xl, engine='openpyxl')
Empty DataFrame
Columns: []
Index: []
```

Though ambiguous as to which engine is actually being used. I think we should be raising if an engine is specified in `read_excel` when the target is a `pd.ExcelFile`, as the latter is a lower level construct and will already have an engine in place.

@simonjayhawkins 