onCandle ({ key, cbGID }, cb) {
    this._registerListener('candle', { 0: key }, Candle, cbGID, cb)
  }
Register a handler to be called with each received candle

@param {object} opts - options
@param {string} opts.key - candle set key, i.e. trade:30m:tBTCUSD
@param {string|number} [opts.cbGID] - callback group id
@param {Function} cb - callback
@see https://docs.bitfinex.com/v2/reference#ws-public-candle
@see WSv2#subscribeCandles
@see WSv2#unsubscribeCandles