Manage features consistency

[jjerphan]

For now, features are defined and used in different places:

• settings.features_names and settings.nb_features where features used are defined globally
• discretization.plot_cubes: where indices of features are hard coded
• extraction_data.extract_molecule where molecule get created (and thus where features get defined)

It would be nicer to have something that would ensure that the mapping of features is consistent in all the code base.

A FeatureManager be a class where one can specify the features are to be used as well as the way they get extracted from the original data. This way, all the information about features (settings.features_names, settings.nb_features, and indices of features) and their processing that are used in different places in the code base could be gathered in such an object that would insure their consistency.

[jjerphan]

Done with a dict from feature names to their index. We don't need more for now.