Add (Lopez-paz, 2017) Metrics: ACC, FWT & BWT

[vlomonaco]

We should add the metrics proposed in (Lopez-paz, 2017) within the beta version of Avalanche.

[ryanlindeborg]

I'd love to work on this issue

[vlomonaco]

Perfect @ryanlindeborg, refer to @AndreaCossu for any problem/doubts you may encounter! :)