Add model tracking to programs

[HenriqueAssumpcao]

This PR allows users to identify which LLM in the ensemble was used to generate a given program. We achieve this by a new attribute to the Program class called model_id, which stores the index of the LLMEnsemble model used to generate the program. Some of the generate methods from the LLMEnsemble class now also return the index of the sampled model. Some minor changes to unit tests were needed. This solves #381.