Experiment _create_object should not be in experiment class

[srcansiz]

The method _create_object in experiment is very complex and it is only used for aggregator and node selection strategy. I propose to remove this method and implement loading objects using class methods of the base classes Aggregator.load and Stragety.load. This will reduced the complexity. The implementation can be similar to Optimizer classes

[ybouilla]

I think the implementation of _create_object can be less complex if we delegate the part of passing training_plan as parameter to methods such as Aggregator.load or Strategy_load.

I don't know if we can do better, because the original attempt was to make possible to load Strategy and Aggregator with custom modules, that could be located outside aggregator or strategies folders. But we may want to remove this feature if we consider it not too relevant