updates to codes workloads: switched to hash tables in the workload--i/o lang plug-in, removed codes_workload_get_info from i/o lang and darshan workloads as this is no longer needed in the bg/p model

Improve the performance of the darshan i/o workload by converting the
linked list of per-rank contexts to a hash table.

Update the output directory argument of the mpi-replay tool to just
specify an already existing directory to perform the workload I/O in.

Modify the mpi-replay tool (located at tests/workload) to spit out
verbose information on each event it replays, if user specifies
-v option. Also, modify the mpi-replay tool to perform no-ops for
each replayed event, if user specifies --noop option. The replay
tool should function properly for all workload types now.

TODO: The tool should offer functionality for initializing the context
of the output replay directory. That is, the tool should first scan
the given workload and create all files and preallocate space for all
I/O ops in the workload. Currently, the replay tool will fail on any
files that are not explicitly created by the workload.

adding the mpi-replay tool to the tests/workload directory for replaying workloads from the codes workload api

(1) adding sample codes example and (2) updating codes best practices document with the CODES example