Support new Avalanche version

[NickLucche]

This new version (ContinualAI/avalanche@4380ebc) includes a refactoring of the RLScenario and RLExperience using the newly defined benchmark stack components.

In order to support it, we must also modify our RLStrategy to inherit from the "BaseTemplate" (https://github.com/ContinualAI/avalanche/tree/master/avalanche/training/templates), this is the newest "hierarchy in town" that just joined the Avalanche training module.

This integration should represent a significant step toward an eventual merge of Avalanche RL into Avalanche.

[NickLucche]

Other significant changes in avl since version 0.0.1 include Plugins (TODO: BasePlugin->RLPlugin) and loggers

[NickLucche]

also we should be installing it from the master branch, the beta version is the one you get when you run pip

[NickLucche]

A solution to this is currently being implemented here https://github.com/ContinualAI/avalanche-rl/tree/new-avalanche-version.
It involves a workaround to keep the old avalanche BaseStrategy around before fully switching to the Templates model.