timeseries review with {fabletools} and extensions

[aspina7]

Consider replacing {trending} given problems discussed in #89, and general package maintenance going forward.

The tidyverts family of packages (which includes {tsibble}) - have extension packages which allow for other models to be used together with {fable}, for example the {fable.tscount} package allows for using {tscount} neg bin and poisson models. (unclear whether the prediction interval is comparable though? need to check).
See wider discussion here.
This would also simplify the cross validation code substantially - with the potential for {caret} integration in the future.

Also check out the new addition of model based outlier() detection