bbw(src, len, mult, _id) {
        let id = this._tsid(_id, `bbw(${len},${mult})`)
        let basis = this.sma(src, len, id)[0]
        let dev = this.stdev(src, len, id)[0] * mult
        return this.ts(2 * dev / basis, id, src.__tf__)
    }
Bollinger Bands Width
@param {TS} src - Input
@param {number} len - Length
@param {number} mult - Multiplier
@return {TS} - New time-series