ENH Datetime64Formatter and Timedelta64Formatter now limit precision.
For Datetime this means that only the date is shown when for all values
there is no timezone and time is midnight and for Timedelta this means
that the deltas are all whole days (GH3401). str(NaT) fixed to be "NaT"
(GH5708).
BUG: Fixed ``to_datetime`` for array with both Tz-aware datetimes and ``NaT``s  (GH5961)