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'

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information