code: def calc_percentiles(self, col_name, where_col_list, where_value_list):
          
          #col_data = self.get_col_data_by_name(col_name)
          col_data = self.select_where(where_col_list, where_value_list, col_name)
          #print('calc_percentiles: col_data = ', col_data, ' where_col_list = ', where_col_list, ', where_value_list = ', where_value_list)
          if len(col_data) == 0:
              #print("Nothing to calculate")
              return 0,0,0
          else:
              first  = self.percentile(col_data, .25)
              third  = self.percentile(col_data, .75)
              median = self.percentile(col_data, .50)
              #print('CALC_PERCENTILES =  first, third, median ',  first, third, median )
              return  first, third, median
docs: calculates the percentiles of col_name 
          WHERE [where_col_list] = [where_value_list]