Finish PauliStringsJax

Created by: inailuig

Stuff which didn't make it into https://github.com/netket/netket/pull/1510 before it was merged. I had done two implementations of the operator (mask and index, and you could do sth hybrid), this

• tests both
• switches the default to indexing, which seems faster for short strings (on gpu one needs to test) In particular with indexing it almost matches the performance of isingjax (convert it with -> to_local_operator->to_pauli_strings->to_jax_operator)
• privately exposes choosing the implementation in the constructor of the operator