replace xarray ufunc by numpy ufunc

This should fix the warning issue in #56 (closed) , although I could not fully test it because I don't get this warning in my environment.

@lleguy

Closes #56 (closed)

doc/changelog.rst

@@ -12,6 +12,7 @@ unreleased
     - fix :issue:`47` SLURM script not using the correct kernel :mr:`176`
     - fix :issue:`51` make sure that BAM units are in ps :mr:`183`
     - fix :issue:`50` and :issue:`54` relating to package dependencies
+    - fix :issue:`56` future warning on xarray.ufuncs :mr:`189`
 
 - **Improvements**