potentially change back IntegratorLearner.ivals from a set to a SortedSet

In d357a64e we change IntegratorLearner.ivals from a SortedSet (sorted by err) to a normal set because the SortedSet was not always actually sorted, bugs (4bed59da), and it was easier to implement.

We should benchmark the IntegratorLearner and see whether it is worth it to change the set back to a SortedSet.