Adding environment variables to choose OpenCL device
Adding environment variables to choose OpenCL device

1. AF_OPENCL_DEFAULT_DEVICE_TYPE
   - Can be one of CPU, GPU and ACC
   - When not set, defaults to first available device
   - Chooses what the default device should be
   - Does not disable other devices

2. AF_OPENCL_DEVICE_TYPE
   - Can be one of CPU, GPU, ACC, ALL
   - When not set defaults to ALL
   - Only chooses devices of given type

bkloppenborg/arrayfire,shehzan10/arrayfire,9prady9/arrayfire,victorv/arrayfire,merlin-ext/arrayfire,ghisvail/arrayfire,arrayfire/arrayfire,9prady9/arrayfire,marbre/arrayfire,bkloppenborg/arrayfire,munnybearz/arrayfire,ghisvail/arrayfire,shehzan10/arrayfire,ghisvail/arrayfire,umar456/arrayfire,bkloppenborg/arrayfire,umar456/arrayfire,shehzan10/arrayfire,arrayfire/arrayfire,merlin-ext/arrayfire,marbre/arrayfire,bkloppenborg/arrayfire,ghisvail/arrayfire,arrayfire/arrayfire,victorv/arrayfire,arrayfire/arrayfire,victorv/arrayfire,umar456/arrayfire,merlin-ext/arrayfire,munnybearz/arrayfire,marbre/arrayfire,munnybearz/arrayfire,9prady9/arrayfire,victorv/arrayfire,marbre/arrayfire,merlin-ext/arrayfire,9prady9/arrayfire,umar456/arrayfire,shehzan10/arrayfire