Modified OpenCL reduce functions for NVIDIA

1. reduce_... functions have been modified to split the work among
threads
2. atomicAdd_g_f3 calls have been replaced by 3xatomicAdd_g_f calls
issued by separate threads

On NVIDIA, for nbnxn_kernel_ElecEw_VdwLJ_F_opencl the execution time
decreased from 20-18ms to 16-14ms