Create a public `.copy()` method that supports changing properties

E.g. for `TimePoint`:
```python-console
>>> a = TimePoint(year=2022, hour_of_day=6)
<TimePoint 2022-01-01T06:00Z>
>>> b = a.copy(hour_of_day=0)
<TimePoint 2022-01-01T00:00Z>
```