BUG: return immediately if memoryview cannot be non-native

late return triggers an assert with python3.4 in debug mode.
Closes gh-4138.
BUG: fix native-only long long check

Caused all long longs to be required native only in memoryviews.
NPY_SIZEOF_LONG_LONG never existed, replace documentation with existing
NPY_SIZEOF_LONGLONG and code with a sizeof(npy_longlong).
BUG DOC TST: Allow __numpy_ufunc__ to handle multiple output ufuncs.

Added details about how the output variables are handled to the spec.
Also adds tests for ufuncs with multiple outputs.
Merge pull request #4170 from juliantaylor/memoryview-error

BUG: core: fix memoryview issues with python 3.4
Merge pull request #4171 from cowlicks/ufunc-override-out

BUG: Allow __numpy_ufunc__ to handle multiple output ufuncs.