stdev(src, len, _id) {

        let sumf = (x, y) => {
            let res = x + y
            return res
        }

        let id = this._tsid(_id, `stdev(${len})`)
        let avg = this.sma(src, len, id)
        let sqd = 0
        for (var i = 0; i < len; i++) {
            let sum = sumf(src[i], -avg[0])
            sqd += sum * sum
        }
        return this.ts(Math.sqrt(sqd / len), id, src.__tf__)
    }
Standard deviation
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series