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.