Merge pull request #891 from ThatDesert/feature/binary-functions

`cf.Data.arctan2()`: Fixes #890 by trying to conform valid units first.

Author: sadielbartholomew

cf/data/data.py

@@ -4407,7 +4407,9 @@ def arctan2(cls, x1, x2):
             x2: array_like
                 X coordinates. *x1* and *x2* must be broadcastable to
                 a common shape (which becomes the shape of the
-                output).
+                output). If both *x1* and *x2* have units, they must
+                be equal or equivalent, in which case they
+                will be conformed.
 
         :Returns:
 
@@ -4434,6 +4436,9 @@ def arctan2(cls, x1, x2):
         [90.0 -90.0]
 
         """
+
+        x1 = conform_units(x1, x2.Units)
+
         try:
             y = x1.to_dask_array()
         except AttributeError: