It should not be possible to create time frames with min=max

[jzernisch]

As long as we don't see a use case for time frames that contain only one point in time, we should not allow to create them, as the are very annoying to work with. The occur when doing intersection operations like

time_frame & time_frame.shift_by(time_frame.duration)

I would prefer to return TimeFrame::EMPTY here.

[pderichs]

👍 regarding your sample.

I would suggest to fail when trying to create a TimeFrame instance with a duration of 0 (min == max).