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 atomicAdd_g_f3 calls have been replaced by 3xatomicAdd_g_f calls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
Added previous OpenCL changes for AMD, nowarp kernels

Added the modified reduce functions also for AMD and nowarp kernels.
For AMD, the following results were obtained:
- FirePro W5100(Windows): the kernel execution time dropped from (36ms,
42ms, 64ms) to (22ms, 34ms, 56ms) as reported in md.log
- For R9-290 OC: the main kernel execution time dropped from 5.6ms to
4.2ms
Fixed incorrect change in gmxManageNvccConfig.cmake
Removed spaces
Cleaned-up, added comments to gpu_utils.cpp
Replaced // TO DO comments with // TODO
Cleaned up, added comments to nbnxn_ocl_data_mgmt files
Merge pull request #70 from StreamComputing/asc

Asc