rma(src, len, _id) {
        let id = this._tsid(_id, `rma(${len})`)
        let a = len
        let sum = this.ts(0, id, src.__tf__)
        sum[0] = this.na(sum[1]) ?
            this.sma(src, len, id)[0] :
            (src[0] + (a - 1) * this.nz(sum[1])) / a
        return sum
    }
Exponentially MA with alpha = 1 / length
Used in RSI
@param {TS} src - Input
@param {number} len - Length
@return {TS} - New time-series