Add 0-sized arrays for particles that will be created

Implements #809 (closed).

Example: If we run with SF we want to have 0-sized star arrays in the snapshots prior to the first star forming event. This helps with all the analysis tools as they don't have to deal with a special case of an hdf group not existing. The arrays have a size (0,1) for most properties and (0,3) where relevant (x, v). The units are correctly written out.

The cell meta-data is also correctly handling the fact that there are genuinely no particles.

Note that the header is untouched and will keep saying there are 0 stars.