GPU halo exchange

GPU halo exchange

Activate with GMX_GPU_DD_COMMS and GMX_USE_GPU_BUFFER_OPS environment
variable.

Class to initialize and apply coordinate buffer halo exchange
functionality directly on GPU memory space.

Currently only supports direct cuda memcpy, and relies on thread MPI
being in use.

Updated gpucomm testing matrices to cover non-GPU case.

Limitation: still only supports thread MPI, 1D data decomposition and
only coordinate halo exchange

Implements part of #2890
Associated with #2915

Change-Id: I8e6473481ad4d943df78d7019681bfa821bd5798