Add an initial snapshot back
I think we should add back an initial snapshot just after reading in the data that outputs:
- Particle IDs
- Particle co-oridnates
- Particle masses
- Particle velocities
as when we construct e.g. gas particles from the DM, or if we ever over-write IDs, the first snapshot we'll have is z=20 which is a bit far from 'initial conditions'
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