text: Title: Portfolio.from_orders(..) for multiple assets
  Body: Following [this doc](https://nbviewer.org/github/polakowo/vectorbt/blob/master/examples/PairsTrading.ipynb#:~:text=%23%20Build%20percentage%20order%20size), I was able to create a list of orders, pass them to Portfolio.from_orders(..) and analyze PnL.
  
  However, I've been struggling with doing the same for multiple assets, e.g. ones downloaded with vbt.YFData.download(SYMBOLS). Are there any examples of how to generate orders for such ohlcv data grouped into hierarchical columns, one per asset?
  
  So far, I have the following, but not sure how convert the result of BBANDS (bb) into orders.
  ```
  SYMBOLS = ['AAPL', 'MSFT']
  data = vbt.YFData.download(SYMBOLS, start=start_date, end=end_date) 
  ohlcv = data.concat()
  bb = vbt.BBANDS.run(ohlcv['Close'])
  bb['AAPL'].middle
  ```
  Thank you!
  
  P.S. I also noticed that `bb['AAPL']['middle']` will throw an error if I replace vbt.BBANDS.run(ohlcv['Close']) with vbt.BBANDS.run(ohlcv['Close'], window=20, ewm=False, alpha=2) which I would expect to behave identically.