onclose(tf) {
        if (!this.env.shared.onclose) return false
        if (!tf) tf = se.tf
        return (se.t + se.tf) % u.tf_from_str(tf) === 0
    }
Returns true when the candle(<tf>) is being closed
(create a new overlay in DataCube)
@param {(number|string)} tf - Timeframe in ms or as a string
@return {boolean}