ENH/BUG: allow single versus multi-index joining on inferred level (GH3662)

[jreback]

WIP on #3662

In [4]: household.join(portfolio, how='inner') Out[4]: male wealth name share household_id asset_id 1 nl0000301109 0 196087.3 ABN Amro 1.00 2 nl0000289783 1 316478.7 Robeco 0.40 gb00b03mlx29 1 316478.7 Royal Dutch Shell 0.60 3 gb00b03mlx29 0 294750.0 Royal Dutch Shell 0.15 lu0197800237 0 294750.0 AAB Eastern Europe Equity Fund 0.60 nl0000289965 0 294750.0 Postbank BioTech Fonds 0.25 [6 rows x 4 columns] In [5]: household.join(portfolio, how='outer') Out[5]: male wealth name share household_id asset_id 1 nl0000301109 0 196087.3 ABN Amro 1.00 2 nl0000289783 1 316478.7 Robeco 0.40 gb00b03mlx29 1 316478.7 Royal Dutch Shell 0.60 3 gb00b03mlx29 0 294750.0 Royal Dutch Shell 0.15 lu0197800237 0 294750.0 AAB Eastern Europe Equity Fund 0.60 nl0000289965 0 294750.0 Postbank BioTech Fonds 0.25 4 NaN NaN NaN NaN 1.00 [7 rows x 4 columns] In [8]: household.index.name='foo' In [9]: household.join(portfolio, how='outer') ValueError: cannot join with no level specified and no overlapping names