text: Title: atr sometimes wrong
  Body: Version: 0.3.02b
  
  **Describe the bug**
  Sometimes I get atr(14) even if I call atr(3)
  
  
  <img width="507" alt="Screenshot_5" src="https://user-images.githubusercontent.com/22365509/124748196-1854eb80-df23-11eb-9d69-069009ac33df.png">
  
  I guess the error is here in atr.py:
  <img width="389" alt="Screenshot_6" src="https://user-images.githubusercontent.com/22365509/124748381-56520f80-df23-11eb-859f-26fa64cab170.png">
  
  probably it has to be:
  atr = ATR(high, low, close, length)
  
  Thanks!
  