Allow inclusion of ongoing events in event time filter

[Flix6x]

In FlexMeasures, I'm planning to move towards using these two new filters to replace the ones we use now, as to include events that are ongoing at the edges of the selected event time window. That is, I intend to replace the use of event_starts_after with event_ends_after, and likewise, to replace the use of event_ends_before with event_starts_before. In FlexMeasures, I am thinking of simplifying the filter names to events_after and events_before (and document that they may include events that partially transpire outside of this time window), while in timely-beliefs, I'd like to keep the explicit names.