neater timezone handling and time corrections in calc_havengetallen

[veenstrajelmer]

We added tz_localize on 29-5-2024 (#30), this means we pass a UTC+1 timeseries as if it were a UTC timeseries. In Deltares/hatyan#333 timezone support was added to astrog. Therefore, we can now conveniently use timezones here also.

So, handle timezones more cleanly, e.g. by not correcting for 1 hour difference in culm_addtime and at the same time dropping a timezone