Added debug buffer for OpenCL kernel - see DEBUG_OCL

Added a new parameter to the OpenCL kernel, a buffer where each thread
can output one float value for debugging purposes.
Host side:
- uncomment #define DEBUG_OCL
This way the host will alloc memory for the debug buffer. Additionally,
after each kernel run it will transfer data back to host and write it to
debug_buffer.txt
Device side:
- call print_to_debug_buffer_f to write to the debug buffer.
Each thread can output one single value.