onTrades ({ symbol, pair, cbGID }, cb) {
    const id = pair || symbol || ''
    const model = id[0] === 'f' ? FundingTrade : PublicTrade

    this._registerListener('trades', { 0: id }, model, cbGID, cb)
  }
Register a handler to be called with each received trade (pair or symbol
required)

@param {object} opts - options
@param {string} [opts.pair] - required if no symbol specified
@param {string} [opts.symbol] - required if no pair specified
@param {string|number} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-public-trades
@see WSv2#subscribeTrades
@see WSv2#unsubscribeTrades