BUG/ER: various panel indexing fixes

- ER: #3777, raise a ``NotImplementedError`` for ``Panel`` -> ``Panel`` setting with alignment (issue will still be open though)
- BUG: closes #3830, panel assignment using loc with a transpose frame did not work
- BUG: closes #4016 fix panel slicing issue that was returning an object that should not have be a reduction in ndim