PME reduction for CUDA F buffer operations

PME reduction for CUDA F buffer operations

Enable with GMX_USE_GPU_BUFFER_OPS env variable.

Provides functionality to perform reduction of PME forces in F buffer
ops kernel.  Currently active when single GPU performs both PME and PP
(multi-GPU support will follow in patch which perfoms PME/PP comms
direct between GPUs). When active, Device->Host copy of PME force
and CPU-side reduction is disabled.

Implements part of #3029, refs #2817

Change-Id: I3e66b6919c1e86bf0bed42b74136f8694626910b