getOB (symbol) {
    if (!this._orderBooks[symbol]) return null

    return new OrderBook(this._orderBooks[symbol])
  }
Returns an up-to-date copy of the order book for the specified symbol, or
null if no OB is managed for that symbol.

Set `managedOrderBooks: true` in the constructor to use.

@param {string} symbol - symbol for order book
@returns {OrderBook} ob - null if not found
@example
  const ws = new WSv2({ managedOrderBooks: true })

  ws.on('open', async () => {
      ws.onOrderBook({ symbol: 'tBTCUSD' }, () => {
        const book = ws.getOB('tBTCUSD')

        if (!book) return

        const spread = book.midPrice()
        console.log('spread for tBTCUSD: %f', spread)
      })

      ws.subscribeOrderBook({ symbol: 'tBTCUSD' })
  })

  await ws.open()