Train an Echo State Network (ESN) to a univariate time series.
   The function automatically manages data pre-processing, reservoir
   generation (i.e., internal states) and model estimation and selection.
   The function is a wrapper for train_esn() and intended to be used
   in combination with fabletools::model().
Arguments
- formula
- Model specification (currently not in use). 
- ...
- Further arguments passed to - train_esn().
