aio-kraken-ws | Get ohlc from kraken web sockets

The callback function is called for each dataset at the end of each dataset’s unit time interval. E.g. if subscription start at 4h42.05 to the dataset ("XBT/EUR", 1), then callback is triggered at 4h43.00, at 4h44.00, at 4h45.00, etc…​ For ("XBT/EUR", 60), it would be at 5h00.00, at 6h00.00, etc…​ It’s possible to get at most 10ms delay between the exact UT interval ending and the actual datetime of the call. If no new data were received from Kraken during an interval, the callback is triggered with the latest known close price and v=0, as it’s described in the following example.