Alternate calendars no longer work due to change in xarray

[arbennett]

Trying to set noleap results in:

  1 Traceback (most recent call last):
  2   File "/glade/u/home/mizukami/anaconda3/envs/metsim/bin/ms", line 10, in <module>
  3     sys.exit(main())
  4   File "/glade/u/home/mizukami/anaconda3/envs/metsim/lib/python3.8/site-packages/metsim/cli/ms.py", line 61, in main
  5     ms = MetSim(setup)
  6   File "/glade/u/home/mizukami/anaconda3/envs/metsim/lib/python3.8/site-packages/metsim/metsim.py", line 218, in __init__
  7     self._times = self._get_output_times(
  8   File "/glade/u/home/mizukami/anaconda3/envs/metsim/lib/python3.8/site-packages/metsim/metsim.py", line 543, in _get_output_times
  9     times = date_range(start + offset, stop + offset + delta,
 10   File "/glade/u/home/mizukami/anaconda3/envs/metsim/lib/python3.8/site-packages/metsim/datetime.py", line 71, in date_range
 11     index = pd.DatetimeIndex(xr.conventions.nctime_to_nptime(times))
 12 AttributeError: module 'xarray.conventions' has no attribute 'nctime_to_nptime'