Use a_nan in testing

[raybellwaves]

We have a_nan in conftest.py (

xskillscore/xskillscore/tests/conftest.py

Line 60 in da06b06

def a_nan(a):

)

but we don't use it everywhere. For example in test_deterministic.py we add nans to the array in the test function (

xskillscore/xskillscore/tests/test_deterministic.py

Line 144 in da06b06

if has_nan:

)

We should update the tests to use a_nan.

We may also want to create a a_nan_dask to test in the dask functions

[raybellwaves]

I'm closing this and the convo can continue at https://github.com/xarray-contrib/xskillscore/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc. Aaron mentioned (can't find the comment) that there are trade offs with uses a_nan in the def test() vs create a nan array on the fly. No best approach.