Particle Packing does a complete copy of a particle array in place

[PhilipDeegan]

Currently for diagnostics and restarts, we copy an entire particle array to SOA to write it to hdf5

it's possible we can do it in chunks, which could be arrays rather than vectors

for SOA particle arrays we do not have to do this copy

but for tiled SOA particle array we may not want to write restarts (or diags) as tiles