Revise `Checkpointer`.

This MR aims at revising the Checkpointer class implemented under declearn.main.utils.

It was initiated by @nbigaud as an extension to his effort to implement Optimizer.get_state and Optimizer.set_state (see MR !19 (merged)). It also aims at handling the newly-added evaluation metrics modularly computed during the FL process (see MR !20 (merged)).

Workflow:

• This MR will start with a review of the (squashed and partially cleaned) implementation made by Nathan.
• Some revisions might be implemented, notably to:
  • Alter the handling of the checkpoint=None case (absence of a folder where to save results).
  • Possibly refactor the code, either within the class, or between Checkpointer, Model and Optimizer.
• A revision of the metrics-saving process will be implemented.
• A revision of the use of the checkpointer in FederatedServer and FederatedClient will be conducted.
• Prospective features and known limitations will be documented into a dedicated issue, for future enhancements.

Tasklist:

• Rework the first draft of the revision done by @nbigaud
• Add metrics-saving and -loading methods compatible with the Metrics API
• Revise Checkpointer instantiation as part of FederatedServer and FederatedClient
• Update FederatedServer and FederatedClient backend to save all metrics