cum(src, _id) {
        let id = this._tsid(_id, `cum`)
        let res = this.ts(0, id, src.__tf__)
        res[0] = this.nz(src[0]) + this.nz(res[1])
        return res
    }
Sum of all elements of src
@param {TS} src1 - Input
@return {TS} - New time-series