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

[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?

[codecov]

Codecov Report

Merging #1481 (2950770) into master (26372e3) will increase coverage by 0.03%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master    #1481      +/-   ##
==========================================
+ Coverage   83.78%   83.81%   +0.03%     
==========================================
  Files         240      241       +1     
  Lines       13687    13720      +33     
  Branches     2076     2079       +3     
==========================================
+ Hits        11467    11500      +33     
  Misses       1706     1706              
  Partials      514      514              

Impacted Files	Coverage Δ
netket/callbacks/earlystopping.py	93.93% <88.88%> (+2.63%)	⬆️
netket/callbacks/__init__.py	100.00% <100.00%> (ø)
netket/callbacks/convergence_stopping.py	100.00% <100.00%> (ø)

[alleSini99]

Maybe a last thing would be to have the possibility to not re-generate the whole sample set at each timestep, but to only update a (controllable) portion of it (using a First In First Out mechanism). This might be useful to reduce the sampling cost.

[gcarleo]

This would be roughly equilavent to using very small number of samples per vmc step and estimate energies and gradients with decaying rolling averages that take into account past values. I think this would be easier to implement just changing the optimizer/ is in the spirit of what Adam and similar would do? The only problem is that we don't have an optimizer that collects decaying running averages for the QGT...

[PhilipVinc]

merging this as the comments are not too relevant and it's useful for infidelity optimisation and similar things..