Indexing, class heirarchies and inheritance

[ieslick]

We need to clarify the behavior (and utility) of class indexing in persistent class heirarchies. Subclasses are often used to differentiate behavior, but also to be able to have siblings and descendants treated as a class. Therefore it is logical that users want to select all descendants of the class in which a slot was first defined. So an index should cover multiple class instances. This needs to be implemented and will likely be very important for the query infrastructure.