Adding in the examples for S3D reads and writes
Adding in the sample xml file for S3D
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.
Attempt to address the scenario where a user poses a read selection over
a transformed variable that intersects no PG. Previously, this would
result in an assertion error, due to the read request never being marked
completed (this flag was set in response to the last pg_read_request
being completed, which never happens since there are 0 PGs).