Fix create_state_model issues related to holiday models.
Fix create_state_model issues related to holiday models.

- Assign the ASIS sampler to the holiday model, instead of the model it holds.

- Changed the cast for the observation model from GaussianModel to
  ZeroMeanGaussianModel, because that is what StateSpaceModel actually uses.

steve-the-bayesian/BOOM,steve-the-bayesian/BOOM,steve-the-bayesian/BOOM,steve-the-bayesian/BOOM,steve-the-bayesian/BOOM