This is the right way to do things in general, but the details get*real* tricky - does the union mesh end up being made of
nearly-arbitrary polyhedra when the input meshes aren't aligned?
I'd want to keep it pretty modular (e.g. ideally System shouldn't need
to know about DTK any more than it knows about Exodus) but I'd love to
see the support get in at the libMesh level.