For now, `frozen` is a strict duplication of `allow_mutation` parameter
i.e. setting `frozen=True` does everything that `allow_mutation=False` does.

NB: this does not change the behavior of `allow_mutation`.

In next commit, setting `frozen=True` will also make the BaseModel hashable
while the existing behavior of `allow_mutation` will not be updated.

Now, setting `frozen=True` also generate a hash function for the model
i.e. `__hash__` is not `None`. This makes instances of the model potentially
hashable if all the attributes are hashable. (default: `False`)

…-immutable-model

Before: there were errors about other stuff than frozen behavior
After: The modification catch only errot related to the frozen behavior

One function tests the behavior: 'the model is mutable'
The other tests the behavior:OC 'the model is immutable'