[Mondrian] descendants of a member

[Anton Nikitin]

We really need a way to optimize DESCENDANTS MDX function in our custom
environment.

Current contract for mondrian.olap.SchemaReader doesn't include any method
to get member descendants (I'm sure it did in some earlier version). Instead
there is mondrian.olap.fun.DescendantsFunDef which calculates descendants by
requesting member children recursively.  

Is it possible to move this functionality into SchemaReader interface in
order to let us overriding and optimizing it?

Anton