Allow heterogenous cell data types?

I've been implementing my own version of Dual Contouring (http://faculty.cs.tamu.edu/schaefer/research/dualcontour.pdf) and I noticed that if all the cell data types returned by refine_data are not identical, then split! fails because the child data gets returned as Tuple{Tuple{...}} instead of just Tuple{...}. Having this ability could be useful to leverage multiple dispatch on the cell data, as well as making the package easier to experiment with. Is there a reason the types need to be identical, or might this even be a bug?

Start by reading the split! implementation and the refine_data entry point, then reproduce the failure with cell data of different types. Determine whether heterogeneous child data can be supported without breaking existing behavior; done means split! handles the requested return shape and has coverage for mixed cell data types.