Add WindowIO and MPIWindowIOManager

[barche]

The objective of this PR is to get some better cooperation between the Julia parallel processing system and MPI.

• WindowIO allows point-to-point communication between MPI processes using a Julia IO. Brief excerpt from the test:

using MPI

MPI.Init()
comm = MPI.COMM_WORLD
const rank = MPI.Comm_rank(comm)
const N = MPI.Comm_size(comm)

const winio = WindowIO(comm) # read from anyone
const writer = WindowWriter(winio, 0) # writes to rank 0
if rank != 0
    write(writer, rank)
    flush(writer) # Must flush to trigger communication
else
    result = read(winio, (N-1)*sizeof(Int)) # Blocks until all required data is read
end

• MPIWindowIOManager sets up a cluster manager to use the WindowIO buffer as communication layer. It adds two modes for start_main_loop:
  • MPI_WINDOW_IO: Normal cluster manager, with the workers only waiting on the process_events loop and all commands only executed on master
  • MPI_WINDOW_NOWAIT: The cluster manager is initialized, but workers don't process events and instead run in SPMD MPI fashion as usual. If a Julia parallel call is needed, it must be prefixed with the @cluster macro to activate the event loops on the workers. All code in the @cluster block then only runs on the master and can contain normal Julia parallel programming calls. After the block ends, SPMD mode is resumed.

[lcw]

I don't know much about the cluster manager but WindowIO seems cool to me.