add statistical models

[m-muecke]

Initial idea was to add the fable instead of the forecast package, but since the interface makes it challenging and yearmonth etc. types are not supported with data.table let's add the forecast package instead.

Update: fable now includes VARIMA and VECM models not supported by the forecast package. Hence, would make sense to use fable instead, due to feature differences.

Packages to add:

• forecast/fable
• smooth

[m-muecke]

Adding {forecast} learners for the initial version, can always add {fable} learners once the API is relatively stable