1. Rewrite argmin and argmax to remove the need of structured arrays.
2. A tile should not apply reducer when data is None.
Merge branch 'master' of https://github.com/fegin/spartan
Sync with new backend.
Can't directly use the update data when the old data is None.
We should apply astype() to the update data to make sure
the update data has the same data type as the old data.
Merge pull request #219 from fegin/master

New backend can't support argmin and argmax. Rewrite them to remove sturctured arrays.