No-modification change
 - set_index(index) sets the index
 - update_index(index) may be used on any index (including other indexes than the index set)
 - insert_in_one_index and build_with_one_index take a boolean 'set_indexes'

We want to be able to update any index (for example a global one) with sequences of a germline,
without changing the index of this germline.