Added partial days support to `DateTime`'s `advance` method.

You can now add partial days (e.g. 2.5.days) to `DateTime` with the advance method.
This was acheived by mimicing the `advance` implementation in `Time`.
Fixed changelog
Merge pull request #12080 from ShayDavidson/fix_datetime_partial_dates

Added partial days support to `DateTime`'s `advance` method.