Various minor fixes

Created by: PhilipVinc

• Make Optimizer a property that can be modified of an optimiser. In that way it can be changed live without having to recreate the optimiser object (which is not nice sometimes, as it means another output file unless you take extra precautions).

• idem for Sr

• For jax Machines, the Sumlayer which takes no argument is a function. I changed it to reflect the jax standard of using them as objects if they do not take any input (this often trips me when mixing netket layers and jax layers).

• Export logCosh and SumLayers

• Fix printing of jax optimisers (there was some dead code). For jax optimisers I can recover their name (inside a try-catch statement, you never know...). otherwise I just print the fully qualified function name.