Add `ConvergenceStopping` callback to stop driver upon convergence to a certain value

Created by: PhilipVinc

Also improve EarlyStopping to support a relative tolerance, which is needed to use it when optimising the infidelity or other quantities converging to zero...

@alleSini99 can you tell me if we need something else to be able to implement the black-box infidelity driver in netket fidelity?