New feature of filter_by_attrs added (#3259)

I missed this in my previous pull request. @dcherian

Author: cspencerjones

doc/whats-new.rst

@@ -78,6 +78,8 @@ Enhancements
   :py:meth:`DataArray.set_index`, as well are more specific error messages
   when the user passes invalid arguments (:issue:`3176`).
   By `Gregory Gundersen <https://github.com/gwgundersen>`_.
+  
+- :py:func:`filter_by_attrs` now filters the coordinates as well as the variables. By `Spencer Jones <https://github.com/cspencerjones>`_.
 
 Bug fixes
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