OpenCL host code - phase -1

0. For main changes:
-> check GMX_USE_OPENCL macro in hw_info.h
-> modify OCL_FILE_PATH with your path to nbnxn_ocl_kernels.cl
1. Added CL folder in src for OpenCL 1.2 headers - to be later removed
2. Added gpu_utils.c for OpenCL functions similar with those from
gpu_utils.cu
3. Renamed some functions from gpu_utils.cu so that it's clear that they
refer to CUDA
4. Added GMX_USE_OPENCL macro in hw_info.h - to be moved to a more
proper location
5. hw_info.h - extended gmx_gpu_info_t, gmx_gpu_opt_t structures to
contain OpenCL related parameters - to be later reviewed and cleaned-up.
6. Added nbnxn_ocl__kernels.cl - convolution kernel from NVIDIA OpenCL
code samples. To be used for testing.