[FEATURE] Ability to fine-tune existing models in a new training run

[mattdawkins]

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Ability to pass in existing trained model as a drop down in the launch training dialogue.

Existing models can either be in the add-on packages, root VIAME, or in any other trained models in the system in probably that priority order.

I'll need to comment on how to pass in flags here and what flags get passed in.

[subdavis]

This is a relatively involved change, could you set priority? Should this be in the next sprint?

[waxlamp]

@mattdawkins could you weigh in on this please?