feat: support mixed indices and column names for dtypes and column selection

[lukapeschke]

Work items:

• Introduced a new type providing information about a column: ColumnInfo
• ExcelSheet.selected_columns now returns a list of ColumnInfo for all selected columns
• ExcelSheet.available_columns now returns a list of ColumnInfo for all available columns
• A lot of to/from python code has been wrapped in traits to have less manual validation

closes https://github.com/ToucanToco/fastexcel/issues/198