The context manager searches for __warningregistry__ entries that
Python leaves all over the place, clearing them, thereby ensuring
that warnings will always be raised. This is necessary for the test
suite to detect warnings even if they were raised before under
the "once" filter.
This PR exposes NumPy 1.8+ padding functionality to all
users of scikit-image, regardless of their personal NumPy
version.
The improved (much better scaling with dimensionality)
version introduced in NumPy 1.8 is used.