Python wrapper for ArrayFire v3.6

Feature parity with ArrayFire v3.6. Refer to the release notes for more information regarding upstream library improvements in v3.6.
- anisotropic_diffusion(): Anisotropic diffusion filter.
- topk(): Returns top-K elements given an array.
Bug fixes:
- Fixed sift() and gloh(), which were improperly calling the library.
Enhancements:
- Added len() method, which returns array.elements().
Documentation:
- Documented statistics API.
- Corrected sign() documentation.
- Modified helloworld example to match C++ lib.

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