Generic prior information over ARMA parameters

[Víctor de Buen Remiro]

It is neccesary to add prior information about ARMA parameters:
Hierarchical linear structure with latent, pure prior and domain restrictions should be added.

Internal mathematical algorithm is trivial due to the only one thing needed is to add log-density of current priors and modify intervals to the intersection with stationarity constraints.

No so easy will be to modify all API's to allow to define it.