Cannot propagate nested groups

[ylorant]

The current version of the group transformer logic doesn't work well with how the propagator works and it causes a crash when trying to propagate a group which is nested inside a transformed group.

The propagator works propaging recursively first, which means that the most nested object gets propaged first. It does it while giving the output's object properties on the nested call to propagate nested values. The problem resides in the fact that with a transformer, you cannot know the final object structure (and thus the proprerties) once the complete object has been processed, ending in trying to access properties on something that is not generated yet.

A solution for that would be to propagate the data in the opposite order, going from the topmost object down to the nested ones, that way the parent mapped object would be there to give out to children groups.