Enforce unique MacroStaticReference for each dynamic model about powsybl-dynawo

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The benefit of current approach is that, when adding a new model, everything is done in the same new class.

We could enforce by design that uniqueness by delegating this role to a MacroStaticReferenceProvider, which would provide a MacroStaticReference based on a Class<? extends BlackBoxModel>, but then we're loosing that aforementioned benefit and the benefit of inheriting getVarsMapping definition when defining a new model which inherits from another.

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Similar issue exists for MacroConnector. For which we also loose the benefit of BusModel / GeneratorModel / LineModel interfaces, which make it easier to define everything needed when adding a new model hence new possible connections.