ENH: Add unique and nunique to GroupBy whitelist

Series.unique and Series.nunique are useful methods for querying the
unique elements (or number of unique elements) in each group when used
in conjunction with DataFrame.GroupBy.  This commit addresses
#6146.

Author: bburan-galenea

pandas/core/groupby.py

@@ -75,7 +75,7 @@
 
 _series_apply_whitelist = \
     (_common_apply_whitelist - set(['boxplot'])) | \
-    frozenset(['dtype', 'value_counts'])
+    frozenset(['dtype', 'value_counts', 'unique', 'nunique'])
 
 _dataframe_apply_whitelist = \
     _common_apply_whitelist | frozenset(['dtypes', 'corrwith'])