harmonise definitions for above/below threshold events

Low flow events were defined as the complement of high flow events,
meaning that Brier scores were symmetric. But it is more consistent to
include the threshold value in both the definitions of low flow and
high flow events since we either study one or the other, and rarely
both at the same time, especially not using the same threshold values.
Plus, the choice of including the threshold in high flow events rather
than low flow ones was really arbitrary.

Note, only the unit tests for contingency table-based metrics are
impacted because threshold-based metrics are using evants="high".