API: add 'level' kwarg for Index.isin method

[immerrr]

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 be None, plain Index objects should only accept None, 0, and, Index.name for the sake of consistency (the last one is especially arguable).

[jreback]

ok, I think if we put this in we can then revert #7867

[jreback]

@cpcloud @jorisvandenbossche thoughts?

[cpcloud]

👍 here, also agree on @immerrr's choice of defaults.

[jorisvandenbossche]

+1