swma(src, _id) {
        let id = this._tsid(_id, `swma`)
        let sum = src[3] * this.SWMA[0] + src[2] * this.SWMA[1] +
                  src[1] * this.SWMA[2] + src[0] * this.SWMA[3]
        return this.ts(sum, id, src.__tf__)
    }
Symmetrically Weighted Moving Average
@param {TS} src - Input
@return {TS} - New time-series