Move to Zarr arrays

We might consider using [Zarr](https://zarr.readthedocs.io/en/stable/index.html) instead of or in parallel to hdf5 if we end up working a lot with stored data and the simulation data allows it: Simpler, [some load-direct-to-gpu support](https://zarr.readthedocs.io/en/stable/user-guide/gpu.html), better support for parallelization.