Add a plugin to manage the weight importance computation and penalization to the loss

[AndreaCossu]

By using a separate plugin to compute importances and penalize the loss we can reuse the same interface for EWC, MAS, SI and all future strategies.

[AntonioCarta]

I started this PR by implementing the interface for LwF (avalanche.training.regularization). Now it can be used easily outside of Avalanche strategies. We can do the same for the other regularization strategies.