max(columnName) {
        return this._castAsNumber(columnName)
            .reduce((p, n) => (n.get(columnName) > p.get(columnName) ? n : p))
            .get(columnName);
    }
Compute the maximal value into a numeric column.
@param {String} columnName The column to evaluate, containing Numbers.
@returns {Number} The maximal value into the column.
@example
df.stat.max('column1')