In the `choose_intermediate_trip_purpose` function, there is an `iloc` assignment [here](https://github.com/ActivitySim/activitysim/blob/1e4ffc5cdd745c183b2b50385e457c8cfaa6c18d/activitysim/abm/models/trip_purpose.py#L74) that now triggers a pandas FutureWarning. This breaks our tests.
