choose datetime object for simulation.from_imod5_data times argument

a review comment on the PR for issue #1068 says:

"I realize we are diverting from the public API. There are a lot of different types of objects in Python to describe time & calendars, which are all still commonly used. See this scheme https://stackoverflow.com/a/21916253

Modflow6Simulation.create_time_discretization accepts a lot of these, and internally converts these to np.datetime64. We use the function imod.util.time.to_datetime_internal() for this. I don't think np.datetime64 has the required year attribute for that. Quickest way would be using the line from to_datetime_internal: year = time.astype("datetime64[Y]").astype(int) + 1970.

It is probably worth a separate issue to align the from_imod5_implementation with create_time_discreatization."