WIP: Refactored CSC __getitem__ to use changes in _unpack_index.

This introduces a failing test due to a bug in CSC's .nonzero() where
the indices are not sorted C-style as described here:
http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#boolean

This addresses @pv comments:
https://github.com/scipy/scipy/pull/2689#discussion-diff-5670362
https://github.com/scipy/scipy/pull/2689#discussion-diff-5670402
DOC: Change comments show test_spares.py is now test_base.py.

Unrelated to current PR.