GROMACS

The GPU traits types include:
- GPU stream (cudaStream_t in CUDA, cl_command_queue in OpenCL)
- GPU context (not used in CUDA, cl_context in OpenCL)
- GPU events (not used in CUDA, cl_event in OpenCL)

These should be made into the opaque types to make the code platform agnostic.

TODO:

1. Use the new structures in:

• PME
• NBNXM
• HaloExchenage

1. Do not create an empty context all the time (e.g. by making the DeviceContext deviceContext_ = DeviceContext{}; field in the UpdateConstraints)
2. Finish the renaming (commandStream -> deviceStream, etc. for fields and local variables)