Create folders when needed, rather than at beginning?

[shinzlet]

Right now we create an empty zarr archive folder structure at the destination before starting softcopy. However this means that, if the user says they're gonna acquire 1000 timepoints but only acquires 3, we have lots of folders on the destination that don't neccessarily exist at the source. Although I'm not sure if tensorstore creates folders on demand either - this might be a moot point.

[shinzlet]

This is actually useful in either case for tensorstore - it takes a long time to create ~10^4 folders on our cluster, and so it would probably be a lot faster to create them on demand