Fix bug in is_discrete_axis call

Author: davidhassell

cf/field.py

@@ -1002,7 +1002,7 @@ def _binary_operation(self, other, method):
             for axis in f.domain_axes(todict=True):
                 identity = None
 
-                if self.is_discrete_axis(axis):
+                if f.is_discrete_axis(axis):
                     # This is a discrete axis whose identity is
                     # inferred from all of its auxiliary coordinates
                     x = {}