text: Title: Sortino ratio of 4,000? 
  Body: This may just be a problem with data, but I am getting blown up sortino ratios. I thought they had to be much smaller. 
  
  For instance: 
  
  ```python
  import yfinance as yf 
  
  msft = yf.Ticker("ETH-USD")
  
  def getSortino(h, p):
    sh = []
    for i in range(0, len(h)):
      if i > p:
        a = h['Close'].iloc[i-p:i]
        sh.append(ta.sortino_ratio(a))
      else: sh.append(0)
  
    return sh
  
  
  h['rso364'] = getSortino(h, 364)
  h['rso100'] = getSortino(h, 100)
  h['rso1000'] = getSortino(h, 1000)
  
  h.plot(y=['rso100', 'rso364', 'rso1000','Close'], linewidth=0.5)
  ```
  
  Gives me a graph like [this](https://www.dropbox.com/s/mjpryr5etumunge/sortino.png?dl=0), I don't believe max values should be in the range of 4,000. Perhaps there is some missing data or something, but not sure what could cause this... 
  
  
  PS: I keep finding this package extremely useful. 