Close #435

• Add a virtual destructor to comms_iface (to invoke std_comms/mpi_comms destructors when std_comms/mpi_comms objects are deleted through a comms_iface pointer).
• Set owns_mpi_comms to false in initialize_mpi_comms as this takes an already initialized comm as an input parameter so comm may better be destroyed by the caller (and no need to destroy if comm is MPI_COMM_WORLD, just calling MPI_Finalize will be sufficient).