User friendly constructor for AR(I)MA models

[SamuelBrand1]

          Yes I agree. I looked at this and couldn't come up with a nice formulation. Agree best for a new issue given that

Originally posted by @seabbs in #438 (comment)

As per the discussion above, we probably want a constructor method for arma/arima model that has the canonical integer arguments: (p,q) and (p,d,q) (respectively).

The tricky aspect here (IMO) is that the packages I've encountered arima inference (e.g. fable) use a frequentist approach and therefore don't have an API for setting priors. Therefore, we'll have to think of the friendliest way for users to specify priors (and probably what a good "default" prior(s) look like) as well as providing the familiar structure numbers.