Remove rbind nesting from {tern} - e.g. tabulate_survival_subgroups

[Melkiades]

Remove rbinds from {tern} when not from different data (I think the example case I got could be done with a split as the filtering variable can be the split)

[shajoezhu]

hi @Melkiades , can you explain a bit more, why is this necessary here? potential breaking change, but thats ok