ENH: downgrade to warning and exit if MPI_Finalized is called twice.

- this can occur when OpenFOAM is run in serial mode, but connects to
  a program that starts/stops MPI on its own.

src/Pstream/mpi/UPstream.C

@@ -216,10 +216,11 @@ void Foam::UPstream::exit(int errnum)
     MPI_Finalized(&flag);
     if (flag)
     {
-        // Already finalized
-        FatalErrorInFunction
-            << "MPI was already finalized" << endl
-            << Foam::abort(FatalError);
+        // Already finalized - warn and exit
+        WarningInFunction
+            << "MPI was already finalized (perhaps by a connected program)"
+            << endl;
+        ::exit(1);
         return;
     }