cog(src, len, _id) {
        let id = this._tsid(_id, `cmo(${len})`)
        let sum = this.sum(src, len, id)[0]
        let num = 0
        for (var i = 0; i < len; i++) {
            num += src[i] * (i + 1)
        }
        return this.ts(-num / sum, id, src.__tf__)
    }
Center of Gravity
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series