confirm hdf5 bufferring is sane for both field stats and particle sampling

For field stats I explicitly set the size of the HDF5 file buffer. I don't see this done for the particle sampling, although it's not at all obvious that it would be needed anyway. In any case, we should make sure we have some sane defaults, in the case of 10 to 64 million particles, and 128 or more MPI processes.