No more types for models.

No more types for models.

Remove the concept of type for a model (CPU, VM, NETWORK, etc).
Explicitly declare the dependencies between models when adding them to
the engine.

Internally, they are organized in a ordered list, respecting the
dependencies described by the user.

Changes in APIs:
s4u_Engine.hpp:
- Delete get_model_list(simgrid::kernel::resource::Model::Type type)
- Modify add_model(std::shared_ptr<simgrid::kernel::resource::Model> model, std::vector<std::string>&& dep_models = {}): added list of dependencies

Model.hpp:
- set_name(), get_name(): add methods to associate a name to the model