Access datetime fields for DataArrays with datetime-like dtypes.

Similar to pandas, fields can be accessed through the .dt attribute for applicable DataArrays:

>>> ds = xarray.Dataset({'time': pd.date_range(start='2000/01/01',
...                                            freq='D', periods=100)})
>>> ds.time.dt
<xarray.core.accessors.DatetimeAccessor at 0x10c369f60>
>>> ds.time.dt.dayofyear[:5]
<xarray.DataArray 'dayofyear' (time: 5)>
array([1, 2, 3, 4, 5], dtype=int32)
  * time     (time) datetime64[ns] 2000-01-01 2000-01-02 2000-01-03 ...

All of the pandas fields are accessible here. Note that these fields are not calendar-aware; if your datetimes are encoded with a non-Gregorian calendar (e.g. a 360-day calendar) using cftime, then some fields like dayofyear may not be accurate.