onOrderBook ({ symbol, prec, len, cbGID }, cb) {
    this._registerListener('orderbook', {
      0: symbol,
      1: prec,
      2: len
    }, OrderBook, cbGID, cb)
  }
Register a handler to be called with each received candle

@param {object} opts - options
@param {string} opts.symbol - book symbol
@param {string} opts.prec - book precision, i.e. 'R0'
@param {string} opts.len - book length, i.e. '25'
@param {string|number} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-public-order-books
@see WSv2#subscribeOrderBook
@see WSv2#unsubscribeOrderBook