atr(len, _id, _tf) {
        let tfs = _tf || ''
        let id = this._tsid(_id, `atr(${len})`)
        let high = this.env.shared[`high${tfs}`]
        let low = this.env.shared[`low${tfs}`]
        let close = this.env.shared[`close${tfs}`]
        let tr = this.ts(0, id, _tf)
        tr[0] = this.na(high[1]) ? high[0] - low[0] :
            Math.max(
                Math.max(
                    high[0] - low[0],
                    Math.abs(high[0] - close[1])
                ),
                Math.abs(low[0] - close[1])
            )
        return this.rma(tr, len, id)
    }
Average True Range
@param {number} len - Length
@return {TS} - New time-series