Group membership tests.

[vibraphone]

There are still some corner cases left in determining whether an entity may be added to a group. For instance, an empty group without either MODEL_DOMAIN or MODEL_BOUNDARY set will be allowed to associate with the attribute. But there are no checks to prevent you from adding the wrong type of entity (from the attribute's point of view) to the group later. Also, the same group might be associated to attributes with mutually exclusive criteria for membership.