demo_timing.py is broken

demo_timing.py doesn't handle writing to existing XML files properly.

The XMLFile class is very simple and doesn't handle things like

if MPI.rank(mpi_comm_world()) == 0:
    f = File(mpi_comm_self(), "timings_aggregate.xml")
    f << t_sum
    f << t_min
    f << t_max
    f << t_avg

properly. XMLFile is set-up to read, or to write just once.

The proper fix is return a handle to an XML objects that can be manipulated, which at destruction writes to file.

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