Date replace and/or get processes

[m-mohr]

After we've added date_shift, there are some more options that seem obviously useful:

• date_replace(date, value, unit) -> date - replace parts of a date, e.g. set a date to the first of a month date_replace("2020-05-13", 1, "day") -> "2020-05-01"
• date_get(date, unit) -> value - get a part of a date, e.g. get the year date_get("2020-05-13", "year") -> 2020 or get the day date_get("2020-05-13", "day") -> 13 (name could also be date_element in accordance to array_element? 😅)

With date_get and date_shift one could emulate date_replace to some degree, but not fully (e.g. can't do the "snap" behavior) so they have both their use cases.

[jdries]

I just had a use case that requires this: for best available pixel compositing, one weighting score is distance to the middle of the compositing period. In the context of a callback, we have access to the time dimension label (date), but we don't necessarily know for which month we are computing a composite.
With date_replace I can do something like:
abs(date_difference(label, date_replace(label, value=15,component="day")))

Seems like we can take the definitions from the PR for an initial implementation!