Updates for OCL - see comments

1. Added OpenCL host implementation for existing CUDA host functions:
- nbnxn_ocl_min_ci_balanced
- ocl_copy_H2D_generic
- ocl_copy_H2D_async
- ocl_copy_H2D
- ocl_free_buffered
- ocl_realloc_buffered
- init_timers
- nbnxn_ocl_init_pairlist
- nbnxn_ocl_upload_shiftvec
- nbnxn_ocl_clear_f
- nbnxn_ocl_init_atomdata
- nbnxn_ocl_init_pairlist
- nbnxn_ocl_init_atomdata
- nbnxn_ocl_upload_shiftvec
- nbnxn_ocl_min_ci_balanced
2. cl_atom_data : changed fields to cl_mem type
3. cl_timers: replaced 2 event fields by 1 event and two time fields