free MPI Communicators on destruction (#134)

make sure that all communicators that have been created during the
runtime of the application are correctly freed again, to avoid memory
leaks.

src/rocshmem.cpp

@@ -422,7 +422,7 @@ __host__ int rocshmem_team_split_strided(
      * not */
     backend->team_tracker.track(*new_team);
   }
-
+  MPI_Comm_free (&team_comm);
   return 0;
 }