code: def _regroup_mergedRDD(self):
          
          myargs = self._myargs
          mykwargs = self._mykwargs
          self._prep_pandas_groupby()
  
          def regroup(df):
              return df.groupby(*myargs, **mykwargs)
  
          return self._mergedRDD.mapValues(regroup)
docs: A common pattern is we want to call groupby again on the dataframes
          so we can use the groupby functions.