text: Title: Good First Issue >> return NaN when valid data points < length parameter
  Body: hi thanks a lot of the great project, been helping me a lot. there is only one thing that has been bothering me here:
  
  - for indicators like rsi(length = 14), seems the program still carry out the calculation even when total non NA datapoints are less than 14,  so the result would be misleading. Is there anyway the program can return NaN when there is less than 14 valid datapoints passed. 
  
  Thank you!
  Michael 