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