Improved GROUP support

[SteveIves]

Because of the way that CodeGen works, GROUP fields are flattened into multiple individual fields. While this is typically OK for single groups, it can quickly become a problem in nested group scenarios, where the names of the flattened fields can quickly become too long.

Once GROUP support is enhanced in GodeGen, we should re-work the way that GROUP fields are presented in data objects, by leveraging nested classes, etc.