Initial MPI aggregation on BP3Serializer using chain method

Podhorszki, Norbertrequested to merge
github/fork/williamfgc/agg_chain into master

Created by: williamfgc

Refactor aggregation into classes Current MPIChain using one extra buffer per process Refactored BPFileWriter for reusability Added Engine Flush function Tests for write and read with 2 substreams

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