Finish implementation of new training config format

This is part of #68 (closed).

!74 (merged) splits the preprocessing and training configs into separate formats (with their own schemas, though training is an extension of preprocessing currently).

It does not yet implement any of the other changes to the training config discussed in #68 (closed).

• Optimizer configuration !99 (merged)
• Per-split transform configuration !100 (merged)
• Maybe simplifications that we discussed previously, regarding loading more of the config file via inheritance, especially from the pre-processing config, rather than duplicating it. !110 (merged)
• Rename the last config file run_xxx/my_model_final_config_file.yml instead of run_000/my_model_training_config_file.yml !108 (merged)