Feature/433 ordering in fedavg

Fixes #433 (closed).

Implementation logic: try to minimize changes, and delegate to #417 (closed) the improvement of the representation of model parameters and weights. One exception to this is the addition of the compute_weights_for_averaging function in strategy, which will become very useful to simplify the implememtation of custom strategies (e.g. for MedicalFolderDataset). I also provide a test that reproduces the bug.

MR description

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