onTicker ({ symbol = '', cbGID } = {}, cb) {
    const m = symbol[0] === 'f' ? FundingTicker : TradingTicker
    this._registerListener('ticker', { 0: symbol }, m, cbGID, cb)
  }
Register a handler to be called on each received ticker

@param {object} opts - options
@param {string} opts.symbol - symbol for tickers
@param {string|number} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-public-ticker
@see WSv2#subscribeTicker
@see WSv2#unsubscribeTicker