Reconsider scopes for IDs / References

[nealkruis]

Instead of scopes being completely user definable, consider:

1. Not allowing scopes to span data groups that don't inherit from the same data group template (e.g., data group instantiations in C++ must inherit from the same class).
2. Allowing sub-scopes within a data group (e.g., if there is a "Surfaces" data group that is instantiated as "floors" and "walls", then each instance could define a different scope.)