test_split_merged_climos.py yields deprecation warnings

[eyvorchuk]

In a fresh python3 venv with only the packages described in README.md installed, the following warnings occur when running pytest tests/test_split_merged_climos.py:

tests/test_split_merged_climos.py::test_existence[hydromodel_gcm_climos]
tests/test_split_merged_climos.py::test_filenames[hydromodel_gcm_climos]
tests/test_split_merged_climos.py::test_metadata_and_time[hydromodel_gcm_climos]
tests/test_split_merged_climos.py::test_dependent_variables[hydromodel_gcm_climos]
  <string>:6: DeprecationWarning: Calling nonzero on 0d arrays is deprecated, as it behaves surprisingly. Use `atleast_1d(cond).nonzero()` if the old behavior was intended. If the context of this warning is of the form `arr[nonzero(cond)]`, just use `arr[cond]`.

These warnings only occur when tiny_hydromodel_gcm_climos.nc is the input file and pytest==5.4.3, but they go away when pytest==3.6.3.