Add tcga study codes as labels

[VanessaBotha]

Refactor:

• Moved database manager to separate file for readability
• Separate function for create dataset split with checks Fixes:
• Fixed confusions between patient id/code in the queries
• Changed folder_id to patient_id in SplitInfo (because patient_ids are inserted not the folder ids) Added:
• Code to add the TCGA study codes as label categories / values to the database and assign them to patients