onOrderUpdate ({ symbol, id, cid, gid, cbGID }, cb) {
    this._registerListener('ou', {
      0: id,
      1: gid,
      2: cid,
      3: symbol
    }, Order, cbGID, cb)
  }
Register a handler to be called on each order update packet

@param {object} opts - options
@param {string} [opts.symbol] - symbol to match
@param {number} [opts.id] - order ID to match
@param {number} [opts.cid] - order client ID to match
@param {number} [opts.gid] - order group ID to match
@param {string} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-auth-orders
@see WSv2#updateOrder