Added forecast intervals to forecast_esn(), forecast.ESN() and plot.forecast_esn(). Forecast intervals are generated by simulating future sample path based on a moving block bootstrap of the residuals and estimating the quantiles from the simulations.
Added plot.esn() to visualize the internal states (i.e., the reservoir).
Added filter_esn() to extract ESN models from a mable.
Added synthetic_data, a dataset with synthetic time series data as tibble.
