GPU Force Halo Exchange

GPU Force Halo Exchange

Activate with GMX_GPU_DD_COMMS environment variable.

Extends GPU Halo exchange feature to provide GPU Force halo exchange
functionality. Does not yet support virial steps, which require an
extra shift force reduction - these are currently performed on the
non-buffer ops / non direct-comm path. Also has same limitations as
coordinate halo exchange.

Performs part of #2890. Future work to improve synchronization towards
a more one-sided scheme (#3092) and to make depenencies more
explicit (#3093)

Change-Id: Ifc23cc8db2655f7258e68b34e7cdc7b71994e1e8