Concat multiple time steps into single Zarr file

[JackKelly]

As per @tv3141's work in https://github.com/tv3141/metoffice_ec2/blob/storage_optimisation_notebook/storage_optimisation/optimize_storage.ipynb

• reduces data read time
• reduces size on disk by 4%

[tomwhite]

We should definitely be doing this so we can read the data back efficiently as a single dataset. I've suggested a way of doing this in #17.

[JackKelly]

Oops, sorry, this is a duplicate of #17! I'll close this for now...