af::topk for OpenCL CPU backend : 
- no memory copy of the data to topk on
- approximately as fast as std partial_sort
- at least f32 data type
- to be included in AF 3.6.0
- for CPU backend only (no need for gpu openCL)
- Linux 64 installer no-gl compatible centos 6
To be delivered on Feb 9th in a installer from AF master + cuda topk