-tb/--time-before-event should influence merging behaviour

[Breakthrough]

Currently, -tb/--time-before-event does not affect event merging, which is inconsistent with -tp/--time-post-event. See #68 for a discussion of where this came up.

The new proposed merging behaviour is as follows:

[Breakthrough]

This should be prioritized now as per the discussion in #192. Even if it complicates the scan pipeline somewhat, the current behaviour is not intuitive and can lead to a lot of headaches for some users.