implement drop down list for zero level model type

[charlotte]

We want to allow users to edit the "type" field for the zero level model component.
(follows on from ticket #596 )

This requires a controlled vocabulary for the different types of models #661
and a method to implement the controlled vacabulary in the questionnaire (this ticket).

[mariepierre]

On ticket #661 we discussed the coherence between the model class (Earth System, Atmosphere Only, ...) the top-level model belongs to and the ensemble of components (component tree) the user is asked to document.

Given that there is no question of modifying dynamically the component tree in the Qnaire, we just thought it could be useful to guide the user filling the Qnaire with an information message (in red), just to let him know that even if he selected an "Atmosphere Only" it is normal that the whole component tree appear, but that he is not supposed to fill-in ocean and sea-ice components.