Fix memory leak in scan related to the input and output storage of the inner function.
Update comment in scan execute/perform.
Change storage clearing to always be done.

Also add patch for numpy API fixes to help people playing with scan_perform.pyx
Add test that the storage is really cleared.
Merge pull request #2124 from abergeron/fix_scan_leak

Fix memory leak in scan related to the input and output storage of the inner function.