Callbacks submodule
Created by: VolodyaCO
This PR implements a callback to stop training if there are no improvements over energy minimisation, similar to EarlyStopping from TensorFlow.
I also copied an example to show how the EarlyStopping class can be used. However, I don't know why import netket; es = netket.callbacks.EarlyStopping()
does not work (callbacks is not a module of netket, as per the error message).
Closes #488 (closed)