xref#7886

It would be nice to be able to write:

s[s.index.isin(['a','b','c'],level='foo')]

instead of

s[s.index.get_level_values('foo').isin(['a','b','c'])]

The former variant is not only shorter but is also potentially more performant: one only has to look up levels in given container and then operate on labels only avoiding instantiation of sub-index completely.

Default value should beNone, plainIndex objects should only acceptNone,0, and,Index.name for the sake of consistency (the last one is especially arguable).