sd(columnName, population = false) {
        return Math.sqrt(this.var(columnName, population));
    }
Compute the standard deviation into a numeric column.
@param {String} columnName The column to evaluate, containing Numbers.
@param {Boolean} [population=false] Population mode. If true, provide the population standard deviation, not the sample one.
@returns {Number} The standard deviation into the column.
@example
df.stat.sd('column1')