add tests for functions

[tmills]

• is_hub_model in train_system.py
• is_cnlpt_model in train_system.py
• is_external_encoder in train_system.py
• cnlp_convert_features_to_hierarchical in cnlp_data.py
• cnlp_preprocess_data in cnlp_data.py
• _build_pytorch_labels in cnlp_data.py
• _build_event_mask in cnlp_data.py
• _reconcile_labels_lists in cnlp_data.py
• _reconcile_output_modes in cnlp_data.py
• _reconcile_columns in cnlp_data.py
• _concatenate_datasets in cnlp_data.py
• len, getitem in cnlp_data.py
• get_labels in cnlp_data.py
• get_unique_labels in cnlp_processors.py
• infer_output_modes in cnlp_processors.py
• get_task_pruned_dataset in cnlp_processors.py (? maybe delete? can't remember why we needed this)
• freeze_encoder_weights in CnlpModel...
• generalize_encoder_forward_kwargs in CnlpModel...