add rocshmem_barrier() (#61)

* add team-barrier implementation

add a team-barrier API and implementation in the IPC and RO conduit.
Clean up some of the logic in the RO Conduit to distinguish between
sync, sync_all, barrier, and barrier_all.

* add team_barrier_tests to functional tests

src/context.hpp

@@ -137,6 +137,8 @@ class Context {
 
   __device__ void barrier_all();
 
+  __device__ void barrier(rocshmem_team_t team);
+
   __device__ void sync_all();
 
   __device__ void sync(rocshmem_team_t team);