ENH: add separate tracking of MPI_Comm_free, MPI_Group_free pending

- permits distinction between communicators/groups that were
  user-created (eg, MPI_Comm_create) versus those queried from MPI.
  Previously simply relied on non-null values, but that is too fragile

ENH: support List<Request> version of UPstream::finishedRequests

- allows more independent algorithms

ENH: added UPstream::probeMessage(...). Blocking or non-blocking

applications/test/parallel-comm2/Make/options

@@ -1,2 +1,4 @@
-/* EXE_INC = */
-/* EXE_LIBS = */
+include $(GENERAL_RULES)/mpi-rules
+
+EXE_INC  = $(PFLAGS) $(PINC) $(c++LESSWARN)
+EXE_LIBS = $(PLIBS)