'Mixed depth' hierarchical column labels
Modifications to column-hierarchical behaviour as per "'Mixed depth' hierarchical column labels" discussion on pystatsmodels.

If column label tuples contain only empty strings at the lower levels, allows the DataFrame to be indexed with only the higher levels. So for example if a column has the hierarchical label ('a','',''), and there are no other columns with 'a' at their top level, then this change allows the column to be pulled out with df['a'], rather than df['a','','']. It also names the resulting series 'a'. 
The use case for this is when some columns have a deeper hierarchy than others, and you want to treat the shallower columns as if the lower levels of the hierarchy weren't there.

If column label tuples contain only empty strings at the lower levels, allows the DataFrame to be indexed with only the higher levels. So for example if a column has the hierarchical label ('a','',''), and there are no other columns with 'a' at their top level, then this change allows the column to be pulled out with df['a'], rather than df['a','','']. It also names the resulting series 'a'. 
The use case for this is when some columns have a deeper hierarchy than others, and you want to treat the shallower columns as if the lower levels of the hierarchy weren't there.

Could I ask you to add a couple of tests? `pandas/tests/test_multilevel.py` would probably be the right place
 Sure, it will take me a short while to figure out the test framework, but as I'm sure you'll attest it's something I should be doing anyway!
 Well, there's not much to it. You already did the hard part which was modifying the code so that your use case works-- so take your test case, put that in a test function, and throw in some asserts that the results are what you expect
 Ah, sorry, earlier commits broke other tests. All tests now pass...except for `test_daterange.TestDateRange.test_with_tzinfo_ambiguous_times`, but that fails for me without my changes.
 thanks Chris, this looks great. I imagine that test is failing because of missing or outdated pytz
 Cheers! Thanks for including it.
