ema(src, len, _id) {
        let id = this._tsid(_id, `ema(${len})`)
        let a = 2 / (len + 1)
        let ema = this.ts(0, id, src.__tf__)
        ema[0] = this.na(ema[1]) ?
            this.sma(src, len, id)[0] :
            a * src[0] + (1 - a) * this.nz(ema[1])
        return ema
    }
Exponential Moving Average with alpha = 2 / (y + 1)
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series