Add strategies for positive predictions (e.g. ProphetPos support)

[candalfigomoro]

In order to force predictions to be positive (this is a common requirement for many business-related time series), the prophet package can be used with the ProphetPos class (see "Approach 5" here: facebook/prophet#1668 (comment)).

It would be nice to add a "ProphetPosModel" to Kats, so we can force predictions to be positive when we use prophet in Kats.

Maybe a positive constraint could also be added to the other models supported by Kats (SARIMA etc.).

[yangbk560]

Thanks for the suggestion and we will keep this in mind for new feature development! Meanwhile, would it make sense to transform the data first (e.g., y' = log(ts)) and build model based on y' then transform it back. In this we can solve the problem of negative forecasts.