bb(src, len, mult, _id) {
        let id = this._tsid(_id, `bb(${len},${mult})`)
        let basis = this.sma(src, len, id)
        let dev = this.stdev(src, len, id)[0] * mult
        return [
            basis,
            this.ts(basis[0] + dev, id + '1', src.__tf__),
            this.ts(basis[0] - dev, id + '2', src.__tf__)
        ]
    }
Bollinger Bands
@param {TS} src - Input
@param {number} len - Length
@param {number} mult - Multiplier
@return {TS[]} - Array of new time-series (3 bands)