[MERGE] branch 'master-models_inheritance-rco'

Modify the implementation of inheritance (with `_inherit`) between models, in
such a way that extensions to a given model are visible to all its children
models.

Inheriting model A to extend it adds the current class to A's model class.
This works like before, except we do not recreate the model class.

Inheriting from A in B adds A's model class as a parent of B's model class, so
that all modifications in A's model class are visible in B's model class.

Moreover, the `BaseModel` class has been introduced as an abstract model
`base`, and is implicitly inherited by all other models.  That model may be
extended to add a feature on all models, or override a method on all models.

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