Count trajectories: let user choose start/end timestamps by seconds

[JuhoErvasti]

Currently the seconds seem to be determined from the time you open the algorithm (not 100% sure), either way they can vary a lot leading to seemingly random results, which is why in #63 I changed it so that the seconds are always set to 0.

See TODO here.

        # TODO: Currently the datetime widget doesn't allow the user to set the seconds
        # so normalize them to 0 seconds, because they are being set seemingly
        # randomly leading to odd results. We need to check if selecting the time by
        # minutes is OK and if not we might need to write a custom widget

[JuhoErvasti]

Minute precision should be sufficient - closing as completed in 7ba46e9