BUG: (GH4016) fix panel slicing issue that was returning an object that should not
     have be a reduction in ndim
BUG: (GH3830), panel assignment using loc with a transpose frame did not work
ER: GH3777, raise a NotImplementedError for Panel -> Panel setting with alignment
Merge pull request #4507 from jreback/panel_index

BUG/ER: various panel indexing fixes