needing threaded mpi for maxThreadFileBufferSize>0

Functionality to add/problem to solve

Currently enabling threaded writing (using maxThreadFileBufferSize > 0) automatically triggers threaded MPI support though in most cases this not needed since the buffer is big enough to avoid additional comms. Problem is that we don't know in advance how much the user is writing.

Using threaded MPI can incur a runtime penalty.

Target audience

Using threaded writing

Proposal

E.g. if user specifies it explicitly (maybe through negating the buffer size specification)

maxThreadFileBufferSize < 0

assume that the buffer space is big enough to write all.

@mark @andy @Prashant