Updates for OCL - see comments

1. Added nbnxn_ocl.c file - the equivalent of nbnxn_cuda.cu
2. Added nbnxn_ocl.h file - the equivalent of nbnxn_cuda.h
3. Added nbnxn_ocl_kernels.clh - the equivalent of
nbnxn_cuda_kernels.cuh
4. Added kernel obj initialization in init_ocl_gpu (needs improvements)
5. ocl_gpu_info_t - added new fields for pointer to kernels

ISSUES
-------
1. Clean-up required for headers included in nbnxn_ocl_kernel_nvidia.clh
See cl build errors if commenting:
__kernel void test
and un-commenting:
__kernel void NB_KERNEL_FUNC_NAME(nbnxn_kernel, _F_cuda_opencl)
2. Some kernel parameters are currently data structures stored on host.
Also the data structures contain pointers. Must be redesigned.
See clEnqueueNDRangeKernel function call for comments.