text: Title: in bbands function stdev does not behave well when there are NaN values in the input column
  Body: when my input columns has NaN values all my standard_deviation values after the NaN inputs become NaN too (even after there are no longer NaN inputs). This should not be the case and only standard_deviation values close to the input NaNs should be impacted. 
  
  I recommend changing this:
  ```standard_deviation = stdev(close=close, length=length, ddof=ddof)```
  
  with this: 
  ```close.rolling(length).std(ddof=ddof)```
  
  and the problems looks resolved while all functionality remains