Enable padding with blanks during expansion of selector, also try expanding selector with only one level into list of bare tokens so as to be compatible with non-MultiIndex DataFrames.
Revert special-case expansion of selectors with one level.
Modify PathLikeSelector.get_tuples() to use updated expand() method and avoid dependency on structure of df.
Add idx_levels property to Interface, fix is_in_interfaces() using new support for specifying padding to PathLikeSelector.expand() method.