Expose use_cftime option in open_zarr

use_cftime was recently added as an option to decode_cf and open_dataset to give users a little more control over how times are decoded (#2759). It would be good if it was also available for open_zarr. This perhaps doesn’t have quite the importance, because open_zarr only works for single data stores, so there is no risk of decoding times to different types (e.g. as there was for open_mfdataset, #1263); however, it would still be nice to be able to silence serialization warnings that result from decoding times to cftime objects in some instances, e.g. #2754.

[max-sixty]

@spencerkclark @dcherian for transparency I unpinned this to make room for the broken docs -> github link. Feel free to unpin the build warnings if this is still top-of-mind

[naomi-henderson]

@Geektrovert and @spencerkclark - any progress on this? It would be great to have it working in time for the CMIP6 hackathon in order to suppress all of the warning messages when opening zarr stores. For example, a zarr store using cftime.DatetimeGregorian (e.g., cftime.DatetimeGregorian(2349, 12, 16, 12, 0, 0, 0, 4, 350)) will always generate the warning:

/usr/local/python/anaconda3/envs/pangeoJun2019/lib/python3.6/site-packages/xarray/coding/times.py:465: SerializationWarning: Unable to decode time axis into full numpy.datetime64 objects, continuing using cftime.datetime objects instead, reason: dates out of range
  dtype = _decode_cf_datetime_dtype(data, units, calendar, self.use_cftime)
/usr/local/python/anaconda3/envs/pangeoJun2019/lib/python3.6/site-packages/xarray/coding/times.py:465: SerializationWarning: Unable to decode time axis into full numpy.datetime64 objects, continuing using cftime.datetime objects instead, reason: dates out of range
  dtype = _decode_cf_datetime_dtype(data, units, calendar, self.use_cftime)
/usr/local/python/anaconda3/envs/pangeoJun2019/lib/python3.6/site-packages/numpy/core/numeric.py:538: SerializationWarning: Unable to decode time axis into full numpy.datetime64 objects, continuing using cftime.datetime objects instead, reason: dates out of range
  return array(a, dtype, copy=False, order=order)

For the CMIP6 collection, I always use use_cftime=True in open_mfdataset, but then when I make the zarr store and read it back in, it generates these warnings - always a distraction for anyone not used to dealing with this data