AggregateDataSet filter runs into MPI size constraint

There have been occasions where an mpi error is encountered where mpi is trying to send a message that is too large. The error looks like "This operation not yet supported for more than bytes".

This error was last encountered with the AggregateDataSet filter, in which Bob Kares was going from 30K procs to 200 procs. I've also encountered this error in the past, too. Bob has got it working now, but I'm not sure what he is doing differently.