Update MEPS npy example

[leifdenby]

I am just writing some notes here with suggestions for what could be done to update the MEPS Npy example:

• Add README based on "Format of data directory" from v0.1.0 of neural-lam README https://github.com/mllam/neural-lam/tree/v0.1.0?tab=readme-ov-file#format-of-data-directory
• Move information about step_length (in number of hours), number of timesteps in forecast and number of ensemble members into config (https://github.com/mllam/neural-lam/blob/main/neural_lam/weather_dataset.py#L58, https://github.com/leifdenby/neural-lam/blob/feat/datastores/neural_lam/datastore/npyfiles/store.py#L160)