cubicweb #1691306 Make orm autogenerated attributes overridable [open]

It's currently not possible for a user-defined entity class to override an orm-based attribute.

Consider the following simple case:

class MyCWUser(AnyEntity):
    __regid__ = 'CWUser'
    def in_group(self):
        return my_super_optimized_version_of_in_group

With our current ORM implementation, the in_group property will never be called. I find this at least confusing for a newcomer since there is no warning this property will be ignored, and I'm tempted to consider this a bug. I actually want to be able to override autogenerated schema-based attributes, especially for optimization concerns since fetch_attrs is sometimes too limited.

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