User Option to Choose Btwn Declarative and Imperative Model Specification

[papamarkou]

The infrastructure is already there to accommodate both declarative and imperative graph-based models (the former is achieved via existing code for topological sorting of the models' variables). Simply add a relevant field in the forthcoming Gibbs job to allow the user to choose between declarative and imperative models.

[papamarkou]

Irrelevant at this point, since only imperative model specification is supported in Klara due to discontinued support of the Graphs package.