Add Outputs base class

[JosePizarro3]

@ndaelman-hu @JFRudzinski

This is the issue to add the basic Outputs section. I will call for review once it is ready.

Some basic ideas:

• It should contain model_system_ref (repeats=False), atoms_state_ref (repeats=True), orbitals_state_ref (repeats=True).
• It should contain is_derived and outputs_ref for when a property is derived during normalization.
• It should contain is_converged and be connected with numerical_settings.SelfConsistency

I leave out handling times for another issue.

[JosePizarro3]

Actually, I am not sure if we need a repeated section for atoms_state_ref or orbitals_state_ref. I guess they should not, as each output property will individually refer to each multi-grain system / individual grain / atom / orbital, and not to the whole group of them.

[JosePizarro3]

Last:

• We need to include the scf steps within the Outputs. I will think on the schema and come with an idea, but I guess we can keep the converged value of each property outside each sfc step, and in the sfc steps just the evolution of such property.

[ndaelman-hu]

Last:

* We need to include the scf steps within the `Outputs`. I will think on the schema and come with an idea, but I guess we can keep the converged value of each property outside each sfc step, and in the sfc steps just the evolution of such property.

Yes, that sounds sensible. The issue with scf, is that most people don't use it. It just gives nice stats and convergence plots :p