Improve database models

[VanessaBotha]

• CategoryEnum train —> fit
• Add nullable=False to columns that cannot be empty
• Change relationship to list if multiple are possible (patient.splits, image.labels, image.caches)
• ImageLabels key/value instead of label_data
• Add unique constraint to split_definition.version
• Add unique constraint to split.split_definition_id and patient_id to ensure patient cannot be assigned multiple categories within one split_definition

[jonasteuwen]

Thanks! Can you see if populate_tcga_db.py still works with these changes? I see at least one occurrence of CategoryEnum.TRAIN there.