Generalize the simulations/experiments for Experiment/Suite

[braybaud]

Entity should carry a children property that will allow to generically assign children to the object so we can have Experiment containing Simulation as children and Suite having Experiment as children.

The children classes should have to redefine the get_children function and could expose a more user-friendly property (simulations for Experiment and experiments for Suite).

We will also create the Suite object as part of this issue.