Iterating through strategy based on a while loop instead of for loop

[kazewong]

Currently the main loop of flowMC simply iterates through the list of strategy. It could be interesting to see while loop based strategy in the future.

Describe the solution you'd like
The easiest way to implement this is probably to do it within a strategy, say in train_model.py instead of iterating through n_epochs one can have it trains until certain criteria is met.