Rework preferred external event selector to use similar methods as superevent manager

We currently have a method update_coinc_far that makes a selection for the preferred external event and then updates the joint FAR in the superevent accordingly. However this is not clear to read and would be hard to improve in the future as is. Instead, we could rework this to be more similar to how the superevent manager chooses a preferred event using keyfunc where multiple attributes are used, going down list until a tie breaker is found.