lowest(src, len, _id) {
        let id = this._tsid(_id, `lowest(${len})`)
        let low = Infinity
        for (var i = 0; i < len; i++) {
            if (src[i] < low) low = src[i]
        }
        return this.ts(low, id, src.__tf__)
    }
Lowest value for a given number of candles back
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series