error in arrayfire.statistics.cov?

`arrayfire.statistics.cov` in version 3.5.20170817 of the Python bindings calls `af_cov_all` via ctypes; however, there doesn't appear to be any such function in the arrayfire library source (at least as of revision [ad230a2](http://github.com/arrayfire/arrayfire/commit/ad230a24c44d8c7526ed9e8138aa6437c5d3cc04)).

Start at arrayfire.statistics.cov and trace its ctypes call to af_cov_all, then compare that symbol with the ArrayFire library source at revision ad230a2. Done means the missing-symbol cause is confirmed and calling cov no longer fails because af_cov_all is unavailable.

索引モデルが issue の本文から書いたものです。