Added new names for pandas isna/notna unary functions

In pandas commit
pandas-dev/pandas@7930202
isna and notna were added as aliases for isnull and notnull.  Those need
to be added to PANDAS_UNARY_FUNCTIONS for xarray datasets notnull to work.
Closes pydata#1663.

xarray/core/ops.py

@@ -42,6 +42,8 @@
 # wrapped in an Variable/DataArray
 NUMPY_UNARY_METHODS = ['astype', 'argsort', 'clip', 'conj', 'conjugate']
 PANDAS_UNARY_FUNCTIONS = ['isnull', 'notnull']
+if pd.__version__ >= "0.21":
+    PANDAS_UNARY_FUNCTIONS.extend(['isna', 'notna'])
 # methods which remove an axis
 REDUCE_METHODS = ['all', 'any']
 NAN_REDUCE_METHODS = ['argmax', 'argmin', 'max', 'min', 'mean', 'prod', 'sum',