Describe the bug
anemoi-datasets create --test changes the grid to 20/20, but this means that masks cannot be applied to test datasets. Errors look like ValueError: could not broadcast input array from shape (162,) into shape (40320,)

To Reproduce
anemoi-datasets create datasets/aifs-rd-tpa-ocda-ilr7-mars-o96-2005-2023-6h-v2-hourly-mean-ocean-sea-ice.yaml ../test.zarr --overwrite --test

URL to sample input data
Input data

Additional context
I'm going to give a PR to fix this. Florian told me the code that does this is only useful for ensemble datasets. So I plan to only change the resolution in --test in that circumstance (i.e. if 'number' or 'grid' are a key in a dict in the 'mars' or 'accumulation' or 'accumulation2' sources).