text: Title: Option to pass "smoothing_level" to ES model
  Body: **Is your feature request related to a current problem? Please describe.**
  Yes, we are using ES model to predict the value of the timeseries, now when the next point is being predicted, it is giving a very high weightage to the last point(s). So say if there was a drop in actual data for the last 2 points only, the predicted value goes down
  
  **Describe proposed solution**
  Exponential smoothing model provides the option to pass "smoothing_level"  bounds which is not in Darts. If we get the option to pass this smoothing_level to ES via darts. It would probably resolve the issue. we can have a cap to restrict the weightage. 
  
  
  