Time decoding/encoding functionality not working

[andersy005]

It appears that #122 may have broken other functionality. For example, when a dataset has decoded time values, resample fails:

cam_chem
<xarray.Dataset>
Dimensions:       (ilev: 33, lev: 32, nbnd: 2, ncol: 48602, time: 12)
Coordinates:
  * lev           (lev) float64 3.643 7.595 14.36 24.61 ... 957.5 976.3 992.6
  * ilev          (ilev) float64 2.255 5.032 10.16 18.56 ... 967.5 985.1 1e+03
  * time          (time) object 2013-02-01 00:00:00 ... 2014-01-01 00:00:00

esmlab.resample(cam_chem, freq='ann')

Please open dataset with `decode_times=False`
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-17-7c31f1f5dede> in <module>
----> 1 esmlab.resample(cam_chem, freq='ann')

/glade/work/abanihi/softwares/miniconda3/envs/analysis/lib/python3.7/site-packages/esmlab/core.py in resample(dset, freq, weights, time_coord_name)
    760 
    761     else:
--> 762         ds = dset.esmlab.set_time(time_coord_name=time_coord_name).compute_ann_mean(weights=weights)
    763 
    764     new_history = f'\n{datetime.now()} esmlab.resample(<DATASET>, freq="{freq}")'

/glade/work/abanihi/softwares/miniconda3/envs/analysis/lib/python3.7/site-packages/esmlab/core.py in set_time(self, time_coord_name, year_offset)
    345                 except Exception as exc:
    346                     print('Please open dataset with `decode_times=False`')
--> 347                     raise exc
    348         self.setup()
    349         return self

/glade/work/abanihi/softwares/miniconda3/envs/analysis/lib/python3.7/site-packages/esmlab/core.py in set_time(self, time_coord_name, year_offset)
    340                         self._ds[self.tb_name],
    341                         units=self.time_attrs['units'],
--> 342                         calendar=self.time_attrs['calendar'],
    343                     )
    344                     self.time_bound.data = tb_data

cftime/_cftime.pyx in cftime._cftime.date2num()

cftime/_cftime.pyx in cftime._cftime._dateparse()

cftime/_cftime.pyx in cftime._cftime._datesplit()

AttributeError: 'NoneType' object has no attribute 'split'