Extract stanfit from model object in Py following cmdstanpy refactoring (#1353)
Extract stanfit from model object in Py following cmdstanpy refactoring (#1353)

* add self.stan_fit in fit methods

* small changes to fix tests

* make changes to remove self.params from backend classes
facebookincubator/prophet,facebookincubator/prophet,facebook/prophet,facebook/prophet,facebook/prophet