Update kraken calculation

freqtrade/exchange/kraken.py

@@ -191,4 +191,4 @@ class Kraken(Exchange):
             df = df[(df['date'] >= open_date) & (df['date'] <= close_date)]
             fees = sum(df['open_fund'] * df['open_mark'] * amount * time_in_ratio)
 
-        return fees
+        return fees if is_short else -fees

freqtrade/optimize/backtesting.py

@@ -492,6 +492,7 @@ class Backtesting:
         sell_candle_time: datetime = sell_row[DATE_IDX].to_pydatetime()
 
         if self.trading_mode == TradingMode.FUTURES:
+            # TODO-lev: Other fees / liquidation price?
             trade.funding_fees = self.exchange.calculate_funding_fees(
                 self.futures_data[trade.pair],
                 amount=trade.amount,