code: def get_mean_error(x1, x2=-1, function="MSE"):
      
      if function == "MSE":
          return MSE(x1, x2)
      elif function == "MAE":
          return MAE(x1, x2)
      elif function == "RMSE":
          return RMSE(x1, x2)
      else:
          raise ValueError('The provided error function is not known')
docs: This function returns desired mean error. Options are: MSE, MAE, RMSE
      
      **Args:**
  
      * `x1` - first data series or error (1d array)
  
      **Kwargs:**
  
      * `x2` - second series (1d array) if first series was not error directly,\\
          then this should be the second series
  
      **Returns:**
  
      * `e` - mean error value (float) obtained directly from `x1`, \\
          or as a difference of `x1` and `x2`