IO ExcelFile New Method
…frame access

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew Added method to ExcelFile class to read all the sheets. This allows the users to access a dictionary of all pd.DataFrames using the .sheets() method and access them more easily. Before, you would need to then call a pd.read_excel(ExcelFile, sheet_name) to access the data in that sheet. 
