Added a function into the ADIOS common layer, adios_pin_timestep, to hack
the ADIOS file such that the timestep is reset within each adios_groupsize call
(since the MPI method updates the time_index in the middle of this function,
and so resetting it must happen within this function).

Also updated the adios_write_all_3D.c code to use this hack. It now appears to
correctly write all PGs into the same timestep.