Address jax.ops.update_at_index deprecation

Created by: PhilipVinc

jax released today a new version which deprecates jax.ops.index_update(arr, indices, new_vals) in favour of arr.at[indices].set(new_vals).

This PR udpates netket to use the new syntax (which has been around for quite some time) so to silence deprecation warnings.

We should try to tag a new release soon-ish.

Also includes some cleanup of pylint warnings