Implemented further database functionality

1. Implemented new Patients and Test Results tables and their methods
2. Populating the test results table with history.csv
3. Discharging a patient
4. Backup method for persisting the in-memory database to disk
5. Unit tests

constants.py

@@ -4,6 +4,7 @@ MLLP_END_CHAR = b"\x1c\x0d"
 
 # Path to load and store the trained Decision Tree model
 DT_MODEL_PATH = "model/dt_model.joblib"
+ON_DISK_DB_PATH = "database.db"
 
 # Map for AKI Label
 LABELS_MAP = {"n": 0, "y": 1}