Experiment will allowing xr.open_mfdataset to use dask for NWPs and Satellite to speed up loading

[JackKelly]

[JackKelly]

At the moment, prepare_ml_data.py takes about 65 seconds per HRVSatellite batch, which is way too slow...

The code currently calls xr.open_mfdataset with chunks=None which I think disables dask.. I'll try other options...

I'll try using Dask now...

[JackKelly]

chunks={} doesn't help (from the xr docs: "chunks={} loads the dataset with dask using engine preferred chunks if exposed by the backend, otherwise with a single chunk for all arrays")

[JackKelly]

Woo! chunks='auto' is much faster! Now about 7 seconds per hrv satellite batch!

[JackKelly]

For reference, I had previously (way back in issue #23) decided to use chunks=None... but maybe dask has improved since then??)