code: public static List<Row> stdDevMeanColumns(DataRowsFacade data, String... columns) {
          return aggregate(data, columns, new String[] {"stddev", "mean"});
      }
docs: Returns the standard deviation
  and mean of the given columns
  The list returned is a list of size 2 where each row
  represents the standard deviation of each column and the mean of each column
  @param data the data to get the standard deviation and mean for
  @param columns the columns to get the
  @return