Update ModelCollection to work with new Model class

[jgallowa07]

Update the ModelCollection class and fit_models() function to work with the refactored v2.0 Model class.

Tasks

• Review ModelCollection implementation
• Update any direct access to old Model internals
• Verify aggregation methods work with new Model parameters
• Test parallel model fitting with fit_models()
• Update doctests/examples
• Write integration tests

Acceptance Criteria

• fit_models() successfully fits multiple models in parallel
• ModelCollection aggregation methods work correctly
• All ModelCollection methods produce correct results
• Tests pass