dev(src, len, _id) {
        let id = this._tsid(_id, `dev(${len})`)
        let mean = this.sma(src, len, id)[0]
        let sum = 0
        for (var i = 0; i < len; i++) {
            sum += Math.abs(src[i] - mean)
        }
        return this.ts(sum / len, id, src.__tf__)
    }
Deviation from SMA
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series