Position buffer ops in CUDA

Position buffer ops in CUDA

On all but search steps the buffer ops transform can now be done on a
CUDA GPU. If PME runs on the same GPU the already uploaded coordinates
will be used as input.

Activate with GMX_USE_GPU_BUFFER_OPS env variable.

Note:
-  waits for X copy on the PME stream to finish, need to implement sync
   point between PME and NB streams (in follow-up).

Implements part of #2817

Change-Id: Ib87dabd74a02727898681249691ac9786b8ac65c