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