Refactored training function. (#481)

[ed728]

What this patch does to fix the issue.

Splits the start_training function into functions in a class. More readability changes will be added later. This commit just moves functionality around. The functions was very large, so a smaller commit than this was not possible. This commit only moves stuff around for ease of reviewing.

Link to any relevant issues or pull requests.

Based on #481 .

More PRs will be added after this.

Testing.

Manual and CI.

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