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/reverse_offload/mpi_transport.cpp

@@ -63,7 +63,10 @@ MPITransport::MPITransport(MPI_Comm comm, Queue* q)
   NET_CHECK(MPI_Comm_rank(ro_net_comm_world, &my_pe));
 }
 
-MPITransport::~MPITransport() {}
+MPITransport::~MPITransport() {
+  if (ro_net_comm_world != MPI_COMM_NULL)
+    NET_CHECK(MPI_Comm_free(&ro_net_comm_world));
+}
 
 void MPITransport::threadProgressEngine() {
   auto *bp{backend_proxy->get()};

src/reverse_offload/mpi_transport.hpp

@@ -169,7 +169,7 @@ private:
 
   std::vector<int> outstanding{};
 
-  MPI_Comm ro_net_comm_world{};
+  MPI_Comm ro_net_comm_world{MPI_COMM_NULL};
 
   std::map<CommKey, MPI_Comm> comm_map{};