Fine-tuning

Support for fine-tuning and the tagging-backend finetune command.

New finetune_model.py script.

Properties n_pretraining_iter and n_finetuning_iter from class MaggotClassifier are available in class MaggotTrainer.

scripts/make_models.jl creates the 20230311and 20230311-0 instances in the models directory.

Branch 20230311 diverges.