Add a restart capability

> CDI Proposal Tier 3: Add a restart capability so that partially trained models can be trained further without starting over; this will enable users to quickly explore two-station model suitability before committing to 24 to 72 hours of model-fitting time.

No file, test, or entry point is named. Start by locating the model-fitting entry point and tracing how partially trained models are represented and resumed. Done should allow users to continue fitting a partial model without starting over and support the stated two-station exploration workflow.

Written by the indexing model from the issue text.