[ENHANCEMENT] Adding new notebook example based on Hyndman book.

[vtoliveira]

I propose to add an example based on visnights data used by Hyndman in his book.

This example is good for some reason:

• It is small, you can see clearly the hierarchical structure.
  • It has simple seasonal, trend components, we can show specific models working such as auto_arima and ETS.

I have done a simple implementation here: https://github.com/vtoliveira/scikit-hts-examples/blob/add-visnights-example/notebooks/visnights.ipynb

I would like to improve that taslking a little more about results, demonstrating other models. But first I think would be better to finish issue in scikit-hts (https://github.com/carlomazzaferro/scikit-hts/issues/14)